Pradip DattaSiemens Healthcare Diagnostics, Tarrytown, New York
INTRODUCTION
Immunoassays are used in clinical laboratories foranalysis of a variety of analytes, including hormones,serum proteins, antibodies to infectious or allergicagents, therapeutic drug monitoring, and drugs ofabuse testing. These immunoassays exhibit high sensi-tivity and a broad analytical range. In the early days ofimmunoassays, the methods were laborious, requiringskilled labor. However, revolutionary developments inimmunoassay automation during the past 20 yearsresulted in fast and effective ways of analyzing manyanalytes, and currently more than 100 immunoassaysare commercially available. Most immunoassay meth-ods use specimens without any pretreatment; are easyto use; and are run on fully automated, continuous,high-throughput, random access systems. These assaysuse very small amounts of sample volumes (10-100 μL); reagents may be stored in the analyzer; mosthave stored calibration curves on the automated ana-lyzer system, often stable for 1 or 2 months; and resultscan be reported in 10�30 min. Immunoassays offer fastthroughput, automated rerun, autoflagging (to alert forpoor specimen quality such as hemolysis), and highsensitivity and specificity. Results can be reporteddirectly into laboratory information systems.Immunoassays measure the analyte concentration in aspecimen by forming a complex with a specific bind-ing molecule, which in most cases is an analyte-specific antibody (or a pair of specific antibodies). Thecomplex generates signal (e.g., sandwich immunoas-say), which is then converted into the analyte concen-tration via a “calibration curve.” The immunoreactionis further utilized in various formats and labels, givinga whole series of immunoassay technologies, systems,
and options. However, like every analytical method,immunoassays suffer from interferences that aredescribed later in this chapter.
IMMUNOASSAY METHODSAND ASSAY PRINCIPLES
Immunoassays are homogeneous or heterogeneousin design, and they have different assay formats anddifferent types of signal generation (Table 6.1).Immunoassays are classified by assay format as eithercompetition or immunometric (commonly referred toas “sandwich”) assays. Competition immunoassayswork best for analysis of small molecules, requiring alimited amount of a single analyte-specific antibodyand labeled analyte. In competition immunoassays, theanalyte in the specimen competes with the labeled ana-lyte for limited antibody binding sites, and the signalis measured either without separation of boundlabeled antigen�antibody complexes from free labeledantigen�antibody complexes (homogenous format) orafter separation of bound from free labeled anti-gen�antibody complexes (heterogeneous format). Onthe other hand, sandwich immunoassays are usedmostly for large molecules, such as proteins or pep-tides, and utilize two different specific antibodies inthe assay design. In sandwich immunoassays, the ana-lyte in the specimen binds to two different antibodiesthat recognize two separate binding sites in the antigenmolecule (different epitopes); one antibody may beconjugated to a solid phase and the other to a label.The bound complex is separated from other assaycomponents by a proper washing protocol, and the rel-evant amount of label produces the signal. The signal
in sandwich assays is directly proportional to the ana-lyte concentration; with low background noise, thistype of immunoassay can be highly sensitive, capableof detecting very low concentrations of the analyte.The signals generated are mostly optical—absorbance,fluorescence, or chemiluminescence.
Depending on the need of separation between thebound labels (labeled antigen�antibody complex) ver-sus free labels, the immunoassays may be homoge-neous or heterogeneous. In the former, the bound labelhas different properties than the free label, and nophysical separation between the two is needed. Forexample, in fluorescent polarization immunoassay(FPIA), the free label (which is a relatively small mole-cule—a few hundred daltons) has different Brownian
motion than when label is complexed to a large anti-body ($140 kDa). This difference in the fluorescencepolarization properties of the label is utilized to quan-tify the analyte from the signal generated [1]. In anothertype of homogeneous immunoassay, an enzyme is thelabel, and the activity of the enzyme may be lost if thelabeled antigen is complexed with an antibody. Thisformat is the basis of the enzyme multiplied immunoas-say technique (EMIT) and cloned enzyme donor immu-noassay (CEDIA) methods [2,3]. In the EMIT method,the label enzyme, glucose-6-phosphate dehydrogenase,is active unless the labeled antigen is bound to the anti-body. The active enzyme reduces nicotinamide adeninedinucleotide (NAD) to NADH, and the absorbance ismonitored at 340 nm. Similarly, in the CEDIA method,two genetically engineered inactive fragments of theenzyme β-galactosidase are coupled to the antigen andthe antibody reagents. When they combine, the activeenzyme is produced, and the substrate—a chromogenicgalactoside derivative—produces the assay signal. In athird commonly used format of homogeneous immuno-assay (turbidimetric immunoassay (TIA)), analytes(antigen) or their analogs are coupled to colloidal parti-cles made of latex, for example [4]. Because antibodiesare bivalent, the latex particles agglutinate in the pres-ence of the antibody. However, in the presence of freeanalyte in the specimen, there is less agglutination andthe resulting turbidity can be monitored as end point oras rate. Another example of homogeneous chemilumi-nescent immunoassay technology is the luminescentoxygen channeling immunoassay (LOCI), in whichthe immunoassay reaction is irradiated with light gen-erating singlet oxygen molecules in microbeads(Sensibeads) coupled to the analyte. When bound to therespective antibody molecule, also coupled to anothertype of bead (Chemibead), Chemibeads react withsinglet oxygen and chemiluminescence signals aregenerated, proportional to the concentration of theanalyte�antibody complex. This technology is used inthe Siemens Dimension Vista automated assay system[5]. In the kinetic interaction of microparticle in solution(KIMS) assay, in the absence of antigen molecules, freeantibodies bind to drug microparticle conjugates form-ing particle aggregates that result in an increase inabsorption, which is optically measured at various visi-ble wavelengths (500�650 nm). When antigen mole-cules are present in the specimen, they bind with freeantibody molecules and prevent formation of particleaggregates, resulting in diminished absorbance in pro-portion to the drug concentration. The On-Line Drugsof Abuse Testings immunoassays marketed by RocheDiagnostics are based on the KIMS format.
In heterogeneous immunoassays, on the other hand,the bound label is physically separated from theunbound label, and the generated signal is measured.
TABLE 6.1 Examples of Various Types of CommerciallyAvailable Immunoassays
64 6. IMMUNOASSAY DESIGN AND MECHANISMS OF INTERFERENCES
ACCURATE RESULTS IN THE CLINICAL LABORATORY
The separation is often done magnetically, where thereagent analyte (or its analog) is provided coupled toparamagnetic particles (PMPs), and the antibody islabeled. Conversely, the antibody may also be pro-vided conjugated to the PMPs, and the reagent analytemay carry the label. After separation and washing, thebound label is reacted with other reagents to generatethe signal. This is the mechanism in many chemilumi-nescent immunoassays (CLIA), in which the label maybe a small molecule that generates a chemiluminescentsignal. Examples of immunoassay systems in whichthe chemiluminescent labels generate signals by chemi-cal reaction are the ADVIA Centaur from Siemens andthe Architect from Abbott [6]. An example in whichthe small label is activated electrochemically is theELECSYS automated immunoassay system from RocheDiagnostics [7].
The label may also be an enzyme (enzyme-linkedimmunosorbent assay (ELISA)) that generates chemilu-minescent, fluorometric, or colorimetric signal depend-ing on the enzyme substrates used. Examples ofcommercial automated assay systems using ELISA tech-nology and chemiluminescent labels are Immulite fromSiemens and ACCESS from Beckman-Coulter [8,9].Another type of heterogeneous immunoassay usespolystyrene particles. If these particles are micro-sizes,the type of assay is called microparticle enhancedimmunoassay [10]. In older immunoassay formats, thelabels were radioactive: radioimmunoassay or RIA.Today, RIA is rarely used due to safety and waste dis-posal issues involving radioactive materials.
The main reagent in the immunoassay is the bindingmolecule, which is most commonly an analyte-specificantibody or its fragment. Several types of antibodies ortheir fragments are used in immunoassays, includingpolyclonal antibodies, which are raised in animals afterthe analyte (as antigen) along with an adjuvant areinjected into the animals. Small-molecular-weight ana-lytes are most commonly injected as conjugates to alarge protein. The animal’s sera is monitored for theappearance of analyte-specific antibodies, and when asufficient concentration of the antibody is reached, theanimal is bled. The serum can be used as the analyte-specific binder in an immunoassay; however, in mostcases, antibodies are purified from serum and used inclinically available assays. Because there are manyclones of the antibodies specific for the analyte, theseantibodies are called polyclonal. In newer technologies,a plasma cell of the animal can be selected to producethe optimum antibody and then can be fused to animmortal cell. The resulting tumor cell grows uncon-trollably, producing only the single clone of the desiredantibody. Such antibodies, called monoclonal antibo-dies, may be grown in live animals or cell culture.There are several benefits of monoclonal antibodies
over polyclonal ones. First, the characteristics of poly-clonal antibodies are dependent on the animal produc-ing the antibodies; if the source individual animal mustbe changed, the resultant antibody may be quite differ-ent. Second, because polyclonal antibodies constitutemany antibody clones, polyclonal antibodies may haveless specificity than monoclonal antibodies [11�13].Sometimes, instead of using the whole antibody, frag-ments of the antibody, generated by digestion of theantibody by peptidases (e.g., Fab, Fab’, or their dimericcomplexes), are also used as reagents.
The other main reagent component of the immuno-assay is the label. There are many different kinds oflabels, generating different kinds of signals. For exam-ple, use of acridinium ester labels, when treated withperoxide, produces chemiluminescent signals. Asdescribed previously, an enzyme may be used as thelabel, which can generate different types of signalsdepending on the substrate used for the enzyme.
Although the immunoassay methods are now widelyused, there are limitations and drawbacks to thesemethods. One of them is the limitation of the bindingmolecule (i.e., the antibody) in terms of its specificity.Many of the endogenous metabolites of the analyte,especially if it is a drug molecule, may have structuralrecognition motifs that are very similar to that of theanalyte. There are also other molecules different fromthe analyte but that produce comparable recognitionmotifs as the analyte. These molecules are generallycalled cross-reactants. When present in the sample,these molecules produce falsely elevated (or, in someinstances, falsely lower) results in the relevant immuno-assay [11�13]. Because monoclonal antibodies are morespecific, immunoassays employing such antibodies inreagents suffer less from cross-reactivity than do poly-clonal antibody-based assays, although this may not bethe case for all analytes. Other components in a speci-men (e.g., bilirubin, hemoglobin, protein, or lipid) mayinterfere in the immunoassay by interfering with theassay signal and thus produce incorrect assay results.A third type of immunoassay interference involvesendogenous human antibodies in the specimen, whichmay interfere with the assay reagent components, assayantibodies, or the antigen labels. Such interference maybe due to the presence of heterophilic antibodies or var-ious human anti-animal antibodies in the specimen.
SPECIMEN TYPES USED INIMMUNOASSAYS
Serum and plasma are the most common types of spe-cimens used in immunoassays. Whole blood specimensmust be used for some analytes, such as the immunosup-pressant drugs (cyclosporine, tacrolimus, sirolimus, and
65SPECIMEN TYPES USED IN IMMUNOASSAYS
ACCURATE RESULTS IN THE CLINICAL LABORATORY
everolimus), although the immunosuppressant drugmycophenolic acid can be monitored in serum orplasma. Urine is the most commonly used specimen indrugs of abuse testing. Urine samples are less frequentlyaffected by hemoglobin or icterus. Turbidity interferenceis possible in urine, but the cause is most likely bacterialgrowth or urate precipitation. Preservatives in urine,such as acetic acid, boric acid, or alkali, may interfere insome urine assays. Cerebrospinal fluid (CSF) specimensare used for monitoring the integrity of the blood�brainbarrier (by analyzing plasma proteins) or infection inCSF. The most common CSF interfering substance isblood contamination with hemolysis. Interference fromturbidity is also possible in such specimens. Other typesof specimens used for immunoassays are saliva, sweat,tears, ascitic and stomach fluids, and bronchial secre-tions. Hair and nail specimens have been used to pro-vide evidence of a longer term history of drug abuse[14,15]. Amniotic fluid, cord blood, meconium, andbreast milk have been used to determine fetal and peri-natal exposure to drugs [16].
EXAMPLES OF IMMUNOASSAYS
Immunoassays are commercially available for avariety of analytes: proteins, hormones, tumor mar-kers, rheumatoid factor, troponin, small peptides, ster-oids, and drugs, including the following:
• Anemia markers: ferritin, folate, vitamin B12
• Autoantibodies: to diagnose and monitorautoimmune diseases
• Cardiovascular disease markers: troponin,B-natriuretic peptide, myoglobin, etc.
etc.• Infectious diseases: antibodies to and antigens from
infectious micro-organisms• Liver disease markers• Therapeutic drug monitoring: antibiotics,
anticoagulation, anticonvulsant, antidepression,anti-inflammation, cardioactive, or neoplastic drugs
• Thyroid markers: T4, T3, thyroid-stimulatinghormone (TSH), etc.
• Tumor markers: cancers of breast, colon, prostate,lung, ovary, etc.
PITFALLS IN IMMUNOASSAYS
Although immunoassays are widely used in theclinical laboratory, they suffer from the following types
of interferences, rendering false-positive or false-negative results:
1. Endogenous interfering components that interferenonspecifically via “matrix effects” or in signalgeneration; for examples, bilirubin, hemoglobin,lipids, and paraproteins may interfere withimmunoassays using serum or plasma specimens.
2. Interferences from the endogenous and exogenouscomponents, which cross-react with the antibodiesused to detect the analyte.
3. System or method-related errors, such as pipettingprobe contamination and carryover.
4. Prozone (or “hook”) effect: Depending on theconcentrations of reagent antibodies used in theassay, very high levels of antigen may reduce theconcentrations of “sandwich” (antibody1�antigen�antibody 2) complexes (which generatethe assay signal), instead forming mostly singleantibody�antigen complexes. The hook effectcauses significant false-negative results.
5. Heterophilic interference is caused by endogenoushuman antibodies in the sample. These antibodiesbind to assay components, generating false results.These interferences may also be caused byautoantibodies, macro-analytes (endogenousconjugates of analyte and antibody), macro-enzymes, and rheumatoid factors.
It is important to recognize the sources of such dis-cordant results and conduct follow-up studies to pro-vide clinically meaningful results. In Chapter 5,interference of endogenous bilirubin and lipemia wasdiscussed along with the effect of hemolysis on clinicallaboratory test results. Many drug metabolites cross-react with the antibody against the parent drug—forexample, the cross-reactivity of carbamazepine-10,11-epoxide, an active metabolite of carbamazepine, withvarious carbamazepine immunoassays. Digoxin immu-noassays suffer from interferences from endogenousdigoxin-like immunoreactive substances as well asfrom a variety of drugs (spironolactone, potassiumcanrenoate, and their common metabolite canrenone)and certain herbal supplements. See Chapter 13 for anin-depth discussion of interferences in therapeuticdrug monitoring assays. In this chapter, the interfer-ence of heterophilic antibodies with various immu-noassays is discussed in detail because this topic is notaddressed in-depth in other chapters.
Interference from Heterophilic Antibodies
Heterophilic antibodies are human antibodies thatinteract with assay antibodies, causing false-positive orfalse-negative results. The heterophilic antibodies are
66 6. IMMUNOASSAY DESIGN AND MECHANISMS OF INTERFERENCES
ACCURATE RESULTS IN THE CLINICAL LABORATORY
polyclonal and heterogeneous in nature, and they com-prise the following types: (1) heterophilic antibodies,which interact poorly and nonspecifically with theassay antibodies; (2) anti-animal antibodies, whichinteract strongly and specifically with the assay antibo-dies; (3) autoantibodies, which are endogenous humanantibodies that interfere with an assay; (4) therapeuticantibodies, which are antibodies given therapeuticallythat interfere with an assay; (5) macro-analytes ormacro-enzymes, which are oligomeric or polymericconjugates of an analyte and/or conjugated withendogenous antibody; and (6) rheumatoid factors.Heterophilic antibodies may arise in a patient inresponse to exposure to certain animals or animal pro-ducts, due to infection by bacterial or viral agents, ornonspecifically.
Although many of the immunoglobulin (Ig) clonesin normal human serum may display anti-animal anti-body properties, only those antibodies with sufficienttiter and affinity toward the reagent antibody used inassay may cause clinically significant interference.Among the anti-animal antibodies, the most commonare human anti-mouse antibodies (HAMA) because ofthe wide use of murine monoclonal antibody productsin therapy or imaging. Heterophilic antibody and anti-animal antibody interferences are often classifiedtogether as heterophilic antibody interferences. Suchinterferences have been mostly found with immuno-metric sandwich assays and less often with competi-tion assays. Sample dilution and depletion or removalof interfering antibodies has been recommended toremove heterophilic antibody interference. A patienthistory of exposure to animals or animal products orautoimmune diseases alerts laboratory professionals tothe possibility of encountering heterophilic antibodyinterference in an assay.
Because heterophilic antibodies are found mainly inserum, plasma, or whole blood, but not in urine, suchinterference is absent in analysis of urine specimen forthe same analyte. This provides an excellent way todetect the interference for analytes that may be presentin both matrices. For example, many case studies withfalse-positive human chorionic gonadotropin (hCG) inserum/plasma have been described in the literature. Inevery case, if β-hCG could have been measured in par-allel urine samples, the false results and the resultingdire consequences could have been easily avoided[17,18].
Heterophilic antibody interference may cause criticalimpact and clinical misjudgment, resulting in unneces-sary follow-up testing and unneeded but potentiallydangerous therapy, leading to significant patient mor-bidity, especially when due to a false-positive hCG(also a cancer marker) measurement in serum withoutinvestigating a parallel urine specimen. The fact that
such interferences may not be suspected from thepatient history, or that such an effect may be transientin nature, complicates the responsibility of the clinicallaboratory to report accurate patient results.
Mechanism of Heterophilic Antibody Interference
The interfering antibodies exert their effect by inter-acting with the assay antibody/antibodies or the ana-lyte. In many cases, especially with heterophilicantibodies, the interactions among interfering antibo-dies and assay antibodies are nonspecific and withlower avidity. In such cases, if the assay employs equi-librium conditions, no interference is found. However,most immunoassays are performed on automated ana-lyzers. Because clinicians demand increasingly fasterlaboratory results, most autoanalyzers use immunoas-says under nonequilibrium conditions and thus aremore prone to assay interferences.
In the sandwich-type immunoassays, heterophilicantibodies can form the “sandwich complex” even inthe absence of the target antigen, generating mostlyfalse-positive results. However, if the interfering anti-body binds with only one of the assay reagent antibo-dies (capture or label), false-negative results areobserved, although less frequently than false-positiveresults. In general, competition immunoassays are lessaffected by antibody interference. Two-site, immuno-metric sandwich assays (e.g., cardiac troponin I andhuman chorionic gonadotropin) are more affected byheterophilic antibody interference.
Interference from Heterophilic Antibody
Heterophilic antibodies are poorly defined humanantibodies that cause interference by noncompetitivebinding mostly to the Fc region of assay antibodies;however, instances of heterophilic antibody binding toother parts of the assay antibody (e.g., idiotope or the“hinge” region) have also been reported. Heterophilicantibodies are found more often in sick and hospital-ized patients, with reported prevalences of 0.2�15%.However, during approximately the past decade, mostcommercial assays have started incorporating blockingreagents against heterophilic antibodies in their assayreagent formulation, reducing interference from het-erophilic antibodies. Although a 2002 study based on aliterature review concluded that the incidence of falseresults arising from interference of heterophilic antibo-dies in various immunoassays is only 0.05% [19], a2005 study reported a prevalence of 0.2�3.7% of het-erophilic antibody interference (measured by assayresponses with and without an interference-blockingreagent) in eight automated tumor marker immunoas-says [20]. Heterophilic antibodies may be present inserum in response to microbial infections in patients[21]. However, interferences from specific anti-animal
67PITFALLS IN IMMUNOASSAYS
ACCURATE RESULTS IN THE CLINICAL LABORATORY
antibodies and complement proteins may often bereported together as heterophilic antibody interference.Because heterophilic antibodies are very heterogeneousin nature, and their concentrations may differ amongindividuals, no blocking-reagent can guarantee 100%protection against such interference. Thus, it is impor-tant to note that although the frequency of heterophilicantibody interference may have been reduced, recentlyreported cases have a large magnitude of interference,affecting the clinical interpretation of test results.There are many examples of incorrect results causedby heterophilic antibodies, including calcitonin [22],thyroid [23], and hormones and tumor markers [24].Examples of false-negative α-fetoprotein (AFP) concen-trations in serum, thus causing incorrect diagnoses insecond-trimester Down syndrome screening, havebeen documented by Mannings et al. [25]. The authorsused a different AFP assay to identify the incorrectresults. Because the false-negative results were cor-rected after storage of the samples at 4 %oC for 1 week,the authors concluded that complement proteins actedas interfering factors.
Interference from Human Anti-Animal Antibodies
Human anti-animal antibodies (HAAA) arise mostoften when the patient is exposed to a specific animalantigen. In the majority of cases, the exposure is fromdiagnostic (e.g., tumor targeted imaging agents) ortherapeutic applications of tumor-specific monoclonalantibodies. Because these antibodies are mostly murinein source, the most prevalent examples of HAAA areHAMA, which interfere with mouse antibody-basedimmunoassays [26,27]. Digibind, used in treating life-threatening digitalis toxicity, is the Fab fragment ofsheep anti-digoxin antibodies. Therapeutic insulinmade from pigs may cause generation of anti-pig anti-bodies in patients. However, currently, most insulinpreparations used in drug therapy are bioengineeredin order to eliminate this problem. Factor VIII, whichis used in therapy, is also prepared from pigs. Manyvaccines are generated in rabbits or chickens (eggs).Anti-animal antibodies may also arise from contactwith animals (e.g., animal husbandry or keeping ofanimals as pets) [28] and the transfer of dietary anti-gens across the gut wall in conditions such as celiacdisease [29].
HAAA can belong to the IgG, IgA, IgM, or, rarely,IgE class. When HAAA are elucidated by animalimmunoglobulins, HAAA can have anti-idiotype oranti-isotype specificity. Anti-idiotype antibodies aredirected against the hypervariable region of the immu-noglobulin molecule, which binds the antigen, andanti-isotype antibodies are directed against the con-stant regions. The anti-idiotype antibodies may againgenerate endogenous anti-anti-idiotype antibodies.
Assuming the “mirror-image” principle of idiotypicantibodies, the anti-anti-idiotype antibody could recog-nize the original analyte and may bind to it. However,in general, most HAAA are anti-isotype antibodies.
The magnitude and duration of HAAA vary widely.Serum concentrations of HAAA range from micro-grams to grams per liter. The HAAA may be transientlasting a few days to months and years [30]. The prev-alence estimates of HAAA, especially HAMA, varywidely from ,1% to 80% among different hospitalizedpatients or outpatients. Several commercial assay kitsare available for estimation of HAMA in human serumor plasma [31]. However, due to the heterogeneousnature of HAMA, a negative HAMA test result doesnot confirm the absence of all types of HAMA in asample.
HAMA interferes mostly with immunoassays thatuse murine antibodies in the assay design. As increas-ingly more monoclonal antibodies (most commonsource is mouse) have been used in commercial immu-noassays, the impact of HAMA interference hasbecome a serious clinical issue. Although HAMA con-centration is usually ,10 μg/mL, HAMA concentra-tions as high as 1000 μg/mL have been reported.Because HAMA arise from exposure of patients tomouse antibodies, cancer patients who may have beenexposed to these antibodies as part of imaging or ther-apeutic agents have higher prevalences of HAMA(40�70%) than other patients. HAMA can be IgG(most common), IgM, IgA, or IgE and can be directedto any part of the monoclonal antibody used in theassay (Fc, Fab, idiotope, etc.). HAMA incidences andconcentrations are increasing with the increased use ofdiagnostic or therapeutic use of monoclonal murineantibodies. The current tendency to use therapeutichumanized antibodies is expected to reverse thistrend.
In addition to mouse, rabbit and goat are also usedto generate assay antibodies. Therefore, like HAMA,immunoassay interference caused by human anti-rabbit (HARA) and anti-goat antibodies has also beendescribed. HARA interference was shown in transthyr-etin, haptoglobin, and C-reactive protein assays [32].
Interference from Rheumatoid Factors
Rheumatoid factors (RFs) are IgM-type antibodiesthat interact with assay antibodies at the Fc area. RFsare present in serum from more than 70% of patientswith rheumatoid arthritis. RFs are also found inpatients with other autoimmune diseases. RF concen-tration increases in infection or inflammation. RF inter-ference follows the same mechanism as interferencefrom other types of antibodies. Therefore, in two-antibody immunometric assays, RFs bridge the captureand label antibodies without involving the antigen and
68 6. IMMUNOASSAY DESIGN AND MECHANISMS OF INTERFERENCES
ACCURATE RESULTS IN THE CLINICAL LABORATORY
generate a false-positive signal, thus spuriously elevat-ing the value of the analyte. In single antibodycompetition-type immunoassays, RFs bind to the assayantibody, preventing its reaction to the label reagentthrough steric hindrance, thus generating false-positiveresults. If RFs are suspected to cause interference, thepatient’s history needs to be examined to determine ifthe RF concentration is expected to be elevated in thepatient’s serum. RF concentration in the serum orplasma can also be measured using commerciallyavailable immunoassays. RFs can be removed from thesample by the many separation steps described later inthis chapter. In a study in which RF interference pro-duced false-positive cardiac troponin I results in animmunometric assay, the authors inactivated RFs byincubating the sample with anti-RF antibody and theinterference was eliminated [33].
Interference from Autoantibodies and TherapeuticAntibodies
Autoantibodies are endogenous patient antibodiesthat bind to either the analyte or one of the reagentsused in the assay. Such antibodies are more commonlyobserved in patients suffering from autoimmune dis-eases. Autoantibodies are often identified by gel filtra-tion separation or polyethylene glycol precipitation ofimmunoreactive components from affected sera.
AUTOANTIBODIES TO THE ANALYTE
The autoantibodies may bind to the analyte-labelconjugate in a competition-type immunoassay, produc-ing false-positive results. Alternatively, they may bindto the analyte in a sandwich assay or competitionassay that uses a label containing an analog of the ana-lyte, giving false-negative results. An example hasbeen described in a cardiac troponin I assay (a markerfor cardiovascular diseases; see Chapter 9) in which anautoantibody directed against troponin caused nega-tive interference [34]. On the other hand, Verhoye et al.[35] reported three patients with false-positive thyro-tropin results that were caused by interference from anautoantibody against thyrotropin. The interfering sub-stance in the affected specimens was identified asautoantibody by gel filtration chromatography andpolyethylene glycol precipitation, followed byimmunoreaction.
MACRO-ANALYTES
Often, the analyte may conjugate with autoantibo-dies to create macro-analytes, which may generateincorrect immunoassay results. For example, macro-amylasemia and macroprolactinemia may produceincorrect results in amylase and prolactin assays,respectively. In macroprolactinemia, the hormoneprolactin conjugates with itself and/or with its
autoantibody to create macroprolactin in the patient’scirculation. The macro-analyte is physiologically inactivebut often interferes with many prolactin immunoassays,generating false-positive prolactin results [36]. Suchinterference may be removed by polyethylene glycol pre-cipitation. Another example of macro-analyte is a false-positive AxSYM troponin I result in an asymptomaticpatient caused by an autoantibody�troponin complex.The interference was not observed in four other troponinI immunoassays. Serial dilution and treatment withmouse serum failed to resolve the discordance, indicat-ing the absence of HAMA interference. Gel filtrationchromatography and polyethylene glycol precipitationstudies identified the immunoreactive component as anIgG�troponin complex [37]. On the other hand, the mea-surement of a macro-analyte of squamous cell carcinomaantigen with IgM was used to assess the risk of hepato-cellular carcinoma in patients with cirrhosis by using animmunoassay targeted to detect the macro-analytes astumor markers [38].
AUTOANTIBODY TO A COMPONENT IN THE
REAGENT
In one example, an endogenous anti-avidin antibodyinterfered with a theophylline assay that used the avi-din�biotin system [39]. In this competition-type immu-noassay, the autoantibody interacted with avidin in thereagent, interfering in complex formation and causingfalse-positive results. Of course, if the assay were sand-wich type, the reduced signal would have caused false-negative results. Autoantibodies to various labeledenzymes (e.g., alkaline phosphatase and peroxidase)have been reported, and these antibodies may interactwith the label enzyme in ELISA, thus falsely elevatingthe results in a competition-type immunoassay.
Therapeutic antibodies are used to bind and inacti-vate toxic components from circulation or directedagainst tumor antigens or immunologic receptors. Forexample, Digibind (Glaxo/Burroughs Wellcome), com-posed of Fab fragments from ovine anti-digoxin anti-bodies, is used to treat life-threatening digoxinoverdoses, and it interferes with most digoxin assays.This interference is removable by ultrafiltration or pro-tein precipitation [40] (see Chapter 13).
HOW TO DETECT AND CORRECTHETEROPHILIC ANTIBODY
INTERFERENCES
If a test result does not correlate with the clinicalpicture, false-positive or false-negative test results canbe suspected, but when false results are subtle and/orplausible, the results could be misleading. For exam-ple, a “normal” result may truly be “abnormal,” and a
69HOW TO DETECT AND CORRECT HETEROPHILIC ANTIBODY INTERFERENCES
ACCURATE RESULTS IN THE CLINICAL LABORATORY
disease state of a patient could be missed due to inter-ference in an immunoassay by heterophilic antibodies.Use of Bayesian logic based on the prevalence of a dis-ease may help identify false results in the diagnosis ofa disease [41]. Various ways of detecting heterophilicantibody and correcting interferences due to the pres-ence of heterophilic antibody in the specimen are sum-marized in Table 6.2.
Assay development scientists incorporate steps suchas sample blanking and robust assay design to mini-mize interferences, including matrix effects arisingfrom protein and other nonspecific constituents in thespecimen. Thus, during the development of multi-plexed cytokine assays, researchers screened for andadded appropriate blockers in the reagents to reduceheterophilic interference [42]. When suspected, theinterfering substance may be removed from the speci-men by specific agents, ultrafiltration, gel filtrationchromatography, precipitation, or centrifugation priorto reanalysis. Alternatively, the specimen may be ana-lyzed by a different method for the same analyte,which is known to be free from such interference.
If a discordant result is suspected to be caused byinterference from some endogenous antibody, the bestpractices to confirm such interference include (1) dilu-tion linearity study with the specimen; (2) examinationof the patient history (exposure to immunogenic ani-mals or animal products and history of hyperactiveimmune system); (3) assaying the sample, if possible,using a different immunoassay utilizing different anti-bodies/reagents; and (4) treating the sample to blockthe interference or remove the interfering antibody,and repeating the assay. This strategy is exemplified ina false-positive TSH result leading to thyroxin over-dose. The incorrect result was traced to RF interferencebecause the sample showed nonlinear dilution.The interference was removed by treating the samplewith a heterophile blocking reagent. In addition, acorrect result was also obtained using a differentimmunoassay [43]. An example of nonlinear dilutionfor specimens with heterophilic antibody interferenceis shown in Figure 6.1, which shows the effect ofsuccessive dilutions of a HAMA-containing sample(spiked with 32 μg/mL of theophylline) versus those
TABLE 6.2 Different Sources of Heterophilic Interference, their Detection, and Reduction
Antibody Detection Reduction
Heterophilic antibody(Weak Interference)
Serial dilution producingnon-linear results
1. Non-specific animal serum ‘Cocktail’ of animal sera2. Blocking agent changes result Serial dilution or blocking agent
Heterophilic antibody*(Strong interference)
Serial dilution producingnon-linear results
1. Serial dilution but preferably by using blocking agent.
FIGURE 6.1 HAMA interference detected by sample serialdilution.
70 6. IMMUNOASSAY DESIGN AND MECHANISMS OF INTERFERENCES
ACCURATE RESULTS IN THE CLINICAL LABORATORY
of a serum-based calibrator for the assay (60 μg/mL) ina theophylline immunoassay using a mouse anti-theophylline antibody. The HAMA sample interfereswith the assay and reads 59 μg/mL when assayedundiluted (false-positive value). After successive dilu-tions (1.3-, 2-, 4-, 6-, and 12-fold with the assay dilu-ent), the interfering antibody is diluted enough so asnot to cause any interference in the assay. The slope ofa line fitted through the lowest three dilutions indi-cates a theophylline concentration of 31.1 μg/mL, closeto the original spike value (Datta et al., unpublisheddata). However, dilutions do not always provide thecorrect analyte value in the sample because ofincreased imprecision in the low end of the assay andbecause of the “matrix effect” between the calibratormatrix and a patient sample.
As described previously, a patient history of anyexposure to animal antibodies, illness, or exposure toanimals should also alert for heterophilic antibodies orHAAA as possible sources for inaccurate results. Atthat time, the assay insert should be examined for thetypes of antibodies and heterophilic antibody blockersused in the assay.
There are various types of commercial or home-brew blockers for heterophilic antibody or HAAA[44,45]. The blocker can be nonimmune animal serum,polyclonal antibody, polymerized IgG, nonimmunemouse monoclonals, or a mixture of monoclonal anti-bodies or fragments of IgG [Fc, Fab, or F(antibody’)2]preferably from the same species used to raise thereagent antibodies. As expected, although the nonspe-cific heterophilic interference can be mitigated by addi-tion of nonimmune serum to the reagents, purifiedIgG, preferably of the same subtype as used in assay,is better than serum in reducing the more specific andstronger binding HAAA.
Several blocking agents are commercially available:Immunoglobulin Inhibiting Reagent (IIR;Biorecalamation), Heterophilic Blocking Reagent (HBR;Scantibodies), Heteroblock (Omega Biologicals), andMAB 33 (monoclonal mouse IgG1) and Poly MAB 33(polymeric monoclonal IgG1/Fab; BoehringerMannheim). IIR is a proprietary formulation of high-affinity anti-animal antibody, and HBR is monoclonalmouse anti-human IgM. A suspected discordant sam-ple (e.g., a sample giving false-positive hCG results)may be separately incubated with the blocker and thenre-assayed [44]. Reinsberg [45] studied the efficacy ofvarious blocking reagents in eliminating HAMA inter-ference. In another example, a clinically discordantfalse-positive serum myoglobin result (where anothercardiac marker concentration, such as troponin I, wasnegative) was attributed to HAMA interference. Theinterference could be removed by the use of HBR [46].Most commercial assay reagents include such blockers,
but due to the heterogeneous nature of the interferingantibodies, no blocker can guarantee success in allsamples.
Nonspecific and weaker heterophilic antibody inter-ferences can even be mitigated by incubating the sam-ple with any nonimmune animal serum (even onedifferent from the source of the assay antibody). Use ofa “cocktail of animal sera” to reduce heterophilic inter-ference has been suggested [31]. Thus, heterophilicinterference in a lutropin immunoassay was reducedequally by adding mouse, sheep, or goat sera to thesample [47]. If the interference is complement medi-ated involving Fc parts of the interfering antibodies,heat inactivation of the sample (56�C for 30 min) mayremove the interference [48]. A limitation of thismethod is that not all analytes can survive such anantibody denaturing process.
As previously mentioned, a general concept of pru-dent assay design is to use Fab or F(ab’)2 fragments ofthe analyte-specific antibody, thereby reducing inter-ference from human anti-isotype antibodies [48].Kuroki et al. [49] used human/mouse chimeric anti-body in a carcinoembryonic antigen assay to reduceHAMA interferences. Another interesting concept is touse chicken antibodies in the assay because to date noheterophilic antibody interference against assays usingchicken antibodies has been reported in the literature[50]. By changing the reaction temperature and allow-ing a longer time to achieve equilibrium, it may bepossible to reduce heterophilic antibody interferencebut not HAAA interference [51]. However, with mod-ern autoanalyzers, changing reaction temperature orreaction time is mostly not possible.
If the previously described methods to correct theinterference do not work or cannot be applied, theinterference can be resolved by removing the interfer-ing substance and re-assaying the clean specimen.A simple solution to remove antibody interference isselective removal of the antibodies from a sample. Thiscan be achieved by selective adsorption of human IgGby a solid phase containing protein A or protein G[52]. However, this does not work if the majority of theinterfering antibodies are of the IgM type. Alternately,the antibody fraction in the sample may be precipi-tated out with a polyethylene glycol reagent (prefera-bly PEG 6000) [35]. Low-molecular-weight analytes, ifthey are not highly protein bound, may be extractedaway from the interfering immunoglobulins by prepa-ration of protein-free filtrates via ultrafiltration or pro-tein precipitation (using trichloroacetic acid,sulfosalicylic acid, or ammonium sulfate). The centrifu-gal ultrafiltration is a fast and relatively easy methodthat uses 10- or 30-kDa cutoff filter membrane ultra-centrifugal cartridges (e.g., Amicon’s Microcon orCentricon). For digoxin assays, this step may not only
71HOW TO DETECT AND CORRECT HETEROPHILIC ANTIBODY INTERFERENCES
ACCURATE RESULTS IN THE CLINICAL LABORATORY
remove antibody interference but also remove interfer-ence from endogenous digoxin-like immunoreactivesubstances (B95% protein bound) or Digibind [53]. Onthe other hand, the harsh conditions of acid precipita-tion may damage many analyte molecules.
CONCLUSIONS
Immunoassays on automated systems are widelyused in today’s clinical laboratories. Various types ofimmunoassays have been developed to analyze alltypes and sizes of antigens. The assays use photomet-ric, luminometric, or fluorometric signals and homoge-neous or heterogeneous reaction types. Serum andplasma are the main specimen types used for immu-noassays. Other types of specimens have also beenused. Immunoassays are used not only on the centrallaboratory analyzers but also on the patient bedsidepoint-of-care systems. Despite the excellent sensitivityand specificity of the immunoassays, they suffer frominterferences: serum constituents, cross-reactants, orendogenous antibodies. There are various ways todetect such interferences and obtain accurate results.
References
[1] Jolley ME, Stroupe SD, Schwenzer KS, et al. Fluorescence polar-ization immunoassay: III. An automated system for therapeuticdrug determination. Clin Chem 1981;27:1575�9.
[2] Hawks RL, Chian CN, eds. Urine testing for drugs of abuse.Rockville, MD: National Institute of Drug Abuse (NIDA).Department of Health and Human Services; NIDA researchmonograph 1986; 73.
[3] Jeon SI, Yang X, Andrade JD. Modeling of homogeneous clonedenzyme donor immunoassay. Anal Biochem 2004;333:136�47.
[4] Datta P, Dasgupta A. A New turbidimetric digoxin immunoas-say on the ADVIA 1650 analyzer is free from interference byspironolactone, potassium canrenoate, and their commonmetabolite canrenone. Ther Drug Monit 2003;25:478�82.
[5] Snyder JT, Benson CM, Briggs C, et al. Development of NT-proBNP, troponin, TSH, and FT4 LOCI(R) assays on the newdimension (R) EXL with LM clinical chemistry system [AbstractB135]. Clin Chem 2008;54:A92.
[7] Forest J-C, Masse J, Lane A. Evaluation of the analytical perfor-mance of the boehringer mannheim elecsys 2010 immunoanaly-zer. Clin Biochem 1998;31:81�8.
[8] Babson AL, Olsen DR, Palmieri T, et al. The Immulite assaytube: a new approach to heterogeneous ligand assay. ClinChem 1991;37:1521�2.
[9] Christenson RH, Apple FS, Morgan DL. Cardiac troponin I mea-surement with the ACCESS immunoassay system: analytical andclinical performance characteristics. Clin Chem 1998;44:52�60.
[10] Montagne P, Varcin P, Cuilliere ML, Duheille J. Microparticle-enhanced nephelometric immunoassay with microsphere-antigen conjugate. Bioconjug Chem 1992;3:187�93.
[11] Datta P, Larsen F. Specificity of digoxin immunoassays towarddigoxin metabolites. Clin Chem 1994;40:1348�9.
[12] Datta P. Oxaprozin and 5-(p-hydroxyphenyl)-5-phenylhydan-toin interference in phenytoin immunoassays. Clin Chem1997;43:1468�9.
[13] Datta P, Dasgupta P. Bidirectional (positive/negative) interfer-ence in a digoxin immunoassay: importance of antibody speci-ficity. Ther Drug Monit 1998;20:352�7.
[14] Pichini S, Pacifici R, Altieri I, Pellegrini M, Zuccaro P.Determination of opiates and cocaine in hair as trimethylsilylderivatives using gas chromatography-tandem mass spectrome-try. J Anal Toxicol 1999;5:343�8.
[15] Palmeri A, Pichini S, Pacifici R, Zuccaro P, Lopez A. Drugs innails: physiology, pharmacokinetics and forensic toxicology.Clin Pharmacokinet 2000;38:95�110.
[16] Dickson PH, Lind A, Studts P, Nipper HC, Makoid M, et al.The routine analysis of breast milk for drugs of abuse in aclinical toxicology laboratory. J Forensic Sci 1994;39:2341�5.
[17] Albersen A, Kemper-Proper E, Thelen MHM, et al. A case ofconsistent discrepancies between urine and blood human chori-onic gonadotropin measurements. Clin Chem Lab Med2011;49:1029�32.
[18] Braunstein G. False positive serum human chorionic gonadotro-pin results: causes, characteristics, and recognition. Am J ObstetGynecol 2002;187:217�24.
[19] Levinson S, Miller J. Towards a better understanding of hetero-phile (and the like) antibody interference with modern immu-noassays. Clin Chim Acta 2002;325:1�15.
[20] Preissner CM, Dodge LA, O’Kane DJ, et al. Prevalence of het-erophilic antibody interference in eight automated tumormarker immunoassays. Clin Chem 2005;51:208�10.
[21] Covinsky M, Laterza O, Pfeifer JD, Farkas-Szallasi T, Scott MG.An IgM antibody to Escherichia coli produces false-positiveresults in multiple immunometric assays. Clin. Chem2000;46:1157�61.
[22] Papapetron PD, Polymeris A, Karga H, et al. Heterophilic anti-body causing falsely high serum calcitonin values. J EndocrinolInvest 2006;29:919�23.
[23] Chin KP, Pin YC. Heterophilic antibody interference in thyroidassay. Intern Med 2008;47:2033�7.
[24] Tate J, Ward G. Interferences in immunoassay. Clin BiochemRev 2004;25:105�20.
[25] Mannings L, Trow S, Newman J, et al. Interferences in theautoDELFIA hAFP immunoassay and effect on second-trimester down’s syndrome screening. Ann Clin Biochem2011;48:438�40.
[26] Miller RA, Maloney DG, McKillop J, Levy R. In vivo effects ofmurine hybridoma monoclonal antibody in a patient withT-cell leukemia. Blood 1981;58:78�86.
[27] Grossman H. Clinical applications of monoclonal antibody tech-nology. Urol Clin N Am 1986;13:465�74.
[29] Falchuck KR, Iselbacher KJ. Circulating antibodies to bovinealbumin in ulcerative colitis and crohn’s disease: characteriza-tion of the antibody response. Gastroenterol 1976;70:5�8.
[30] Kazmierczak SC, Catrou PG, Briley KP. Transient nature ofinterference effects from heterophilic antibodies: examples ofinterference with cardiac marker measurements. Clin ChemLab Med 2000;38:33�9.
[31] Kricka LJl. Human anti-animal antibody interferences in immu-nological assays. Clin Chem 1999;45:942�56.
[32] Benoist JF, Orbach D, Biou D. False increase in C-reactive pro-tein attributable to heterophilic antibodies in two renal trans-plant patients treated with rabbit antilymphocyte globulin. ClinChem 1999;45(5):616�18.
72 6. IMMUNOASSAY DESIGN AND MECHANISMS OF INTERFERENCES
ACCURATE RESULTS IN THE CLINICAL LABORATORY
[33] Dasgupta A, Banerjee SK, Datta P. False positive troponin I inthe MEIA due to the presence of rheumatoid factors in serum.Am J Clin Pathol 1999;112:753�6.
[34] Eriksson S, Halenius H, Pulkki K, et al. Negative interference incardiac troponin I immunoassays by circulating troponin auto-antibodies. Clin. Chem 2005;51:839�47.
[35] Verhoye E, Avd Bruel, Delanghe JR, et al. Spuriously high thy-rotropin values due to anti-thyrotropin antibody in adultpatients. Clin Chem Lab Med 2009;47:604�6.
[36] Kavanagh L, McKenna TJ, Fahie-Wilson MN, et al. Specificityand clinical utility of methods for determination of macro-prolactin. Clin Chem 2006;52:1366�72.
[37] Michielsen ECHJ, Bisschops PGT, Janssen MJW. False positivetroponin result caused by a true macro-troponin. Clin ChemLab Med 2011;49:923�5.
[38] Zulin J, Veggiani GL, Pengo P, et al. Experimental values ofspecificity of squamous cell carcinoma antigen-IgM assay inpatients with cirrhosis. Clin Chem Lab Med 2010;48:217�33.
[39] Banfi G, Pontillo M, Sidoli A, et al. Interference from antiavidinantibodies in thyroid testing in a woman with multiendocrineneoplasia syndrome type 2B. J Clin Ligand Assay1995;18:248�51.
[40] Dasgupta A, Wells A, Datta P. Effect of digoxin fab-antibodyon the measurement of total and free digitoxin by fluorescencepolarization and a new chemiluminescent immunoassay. TherDrug Monit 1999;21:251�5.
[41] Ismail AAA, Ismail AA, Ismail Y. Probabilistic bayesian reason-ing can help identifying potentially wrong immunoassaysresults in clinical practice: even when they appear “not-unrea-sonable”. Ann Clin Biochem 2011;48:65�71.
[42] Martins T, Pasi B, Litwin C, Hill H. Heterophile antibody inter-ference in a multiplexed fluorescent microsphere immunoassayfor quantitation of cytokines in human serum. Clin VaccineImmunol 2004;11:325�9.
[43] Georges A, Charrie A, Raynaud S, et al. Thyroxine overdosedue to rheumatoid factor interference in thyroid stimulatinghormone assay. Clin Chem Lab Med 2011;49:873�5.
[44] Butler SA, Cole LA. Use of heterophilic antibody blocking agent(HBT) in reducing false-positive hCG results. Clin Chem2001;47:1332�3.
[45] Reinsberg. Different efficacy of various blocking reagents toeliminate interferences by HAMA in a 2-site immunoassay.Clin Biochem 1996;29:145�8.
[46] Bonetti A, Monica C, Bonaguri C, et al. Interference by hetero-philic antibodies in immunoassay: wrong increase of myoglobinvalues. Acta Biomed 2008;79:140�3.
[47] Sampson M, Ruddel M, Zwig M, Elin RJ. Falsely high concen-tration of serum lutropin measured with the abbott Mx. ClinChem 1994;40:1976.
[48] Vaidya HC, Beatty BJ. Eliminate interference from heterophilicantibodies in a two-site immunoassay for CKMB by using F(ab’)2 conjugate and polyclonal mouse IgG. Clin Chem1992;38:1737.
[49] Kuroki M, Matsumoto Y, Arakawa F, et al. Reducing interfer-ence from heterophilic antibodies in a two-site immunoassayfor carcinoembryonic antigen (CEA) by using a human/mousechimeric antibody to CEA as the tracer. J Immunol Methods1995;180:81�91.
[50] Span PN, Grebenchtchikov N, Geurts-Moespot J, Sweep CGJ.Screening for interference in immunoassays. Clin Chem2003;49:1708�9.
[51] Miller JJ, Valdez R. Approaches to minimizing interference bycross-reacting molecules in immunoassays. Clin Chem1991;37:144�53.
[52] Liendo C, Ghali JK, Graves SW. A new interference in somedigoxin assays: anti-murine heterophilic antibodies. ClinPharmacol Ther 1996;60:593�8.
[53] Dasgupta A, Biddle DA, Wells A, Datta P. Positive andnegative interference of the Chinese medicine Chan Su inserum digoxin measurement—Elimination of interference byusing a monoclonal chemiluminescent digoxin assay or moni-toring free digoxin concentration. Am J Clin Pathol 2000;114:174�9.
