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Diagnostic tests
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A diagnostic test is any kind ofmedical test
performed to aid in the diagnosis or detection
ofdisease
http://en.wikipedia.org/wiki/Medical_testhttp://en.wikipedia.org/wiki/Medical_diagnosishttp://en.wikipedia.org/wiki/Diseasehttp://en.wikipedia.org/wiki/Diseasehttp://en.wikipedia.org/wiki/Medical_diagnosishttp://en.wikipedia.org/wiki/Medical_test7/28/2019 Diagnostic Tests 06.12.2011
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Helps to understand
Patients health
Physiological functioning
Results are reported with a reference range
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Why interpretation?
Appropriateness of patients current drug
therapy
Adequacy of drug response
Efficacy of treatment
Serious drug toxicity
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Medications may alter laboratory test values
through a variety of pharmacological, physical
or chemical mechanisms.
Pharmacologically, a drug may cause a true
change in the concentration of the particular
substance being measured.
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Chemically, a drug or its metabolite may
interfere with the accurate determination of
the desired constituent.
In addition to chemical interferences by drugs,
one should also consider the effects of
anticoagulants and of marked changes in
concentrations of normally occurringsubstances in body fluids which alter
laboratory test values.
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Chemical interferences arise primarily from
lack of specificity of the methods used to
measure desired constituents
Another problem is to determine whether a
reported drug interference was observed at
realistic concentrations which are likely to
occur in serum or urine during a typicaltherapeutic regimen.
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Renal function tests
Serum urea(3-8 mmol/L):
Catabolism of amino acids
Undergoes tubular reabsorption
Vary with diet, rate of protein metabolism, liver
production and GFR.
Upto 10 mmol/L
Non-renal and renal factors influence the increaseand decrease in the levels.
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Uric acid(0.15-0.47 mmol/L):
End product of purine metabolism
Increased rate of formation or reduced excretion
Gouty arthritis
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Drugs interfering with uric acid
measurementInterfering agent
Ascorbic acid
Phenacetin
Caffeine
ChlorineErgothioneine
Salicylates
Levodopa
Methyldopa
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Serum creatinine(55-120 mmol/L or 0.6-1.35
mg/dL):
Product of muscle metabolism
Surrogate marker for GFR
Increase: insult to the kidney
Decrease: no pathological significance
Estimation of GFR
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GFR:
Glomerular filtration rate (GFR) describes the
flow rate of filtered fluid through the kidney
Estimated GFR= (140-age) x weight
Serum creatinine mmol/l x0.84
120-150ml/min
Helps to monitor drug action, transplant
Poses a problem (muscle mass and consumption
of diet)
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Measurement of alternate agent i.e., cystatin
C
Equations are developed considering the
same.
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Drugs interfering with creatinine
measurementInterfering agent Concentration equivalent
Acetoacetic acid. IOO mg/dl of acetoacetic acid =3.4 mg/dl ofcreatinine.
Acetone 500 mg/dl of acetone =1.4
mg/dl of creatinine.
Aminohippurate
Ascorbic acid
Fructose IOOOm g/d1 of glucose
=1 mg/dl of creatinine.
Glucose IOOOm l/d1 of glucose
=0.2 mg/dl of creatinine.
Methyl dopa IO mg/dl of methyldopa = 1.0 mg/dl of creatinine.
Nitrofuran derivatives
Phenolsulphtalein
Pyruvate 10 mgldl of pyruvic acid = 0.3 mgidl of creatinine.
Protein 5 g/d1 of crystalline human serum albumin =2.2 mg/dl
of creatinine.
Sulphbromthalein
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Blood urea nitrogen(2.9-7.1 mmol/L or 8-20
mmol/L):
Alongwith serum creatinine, is very useful
Another prognostic marker for kidney disease
is an elevated level of protein in the urine. The
most sensitive marker of proteinuria is
elevated urine albumin
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Drugs interfering with blood urea nitrogen
measurementInterfering agent
Citrulline
Hydantoin
MethylureaPhenylurea
Sulfonylureas,
Acetone
Chloral hydrate
Creatinine
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Drugs interfering with urobilinogen
measurement
Interfering agent
P-amino salicylate
Chlorpromazine
Bilirubin
Sulphonamides
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SERUM ELECTROLYTES
1. Serum sodium(135-145 mmol/l)
Hyponatraemia: not enough sodium (salt) in the body fluids
Lethargy, nausea, drowsiness and confusion
Pseudohyponatremia, clinical fluid overload, diuretic
therapy, salt wasting nephropathy, The syndrome of
inappropriate antidiuretic hormone hypersecretion
(SIADH)
Acute or chronic
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Hypernatraemia
Excessive loss of hypotonic fluids, severe vomitting
/diarrhea
Diabetes mellitus, acute renal failure, diabetes insipidus Address the underlying dehydration
Causes CNS pathology
Controlled by thirst reflex, ADH, renin-angiotensin-
aldosterone system and renal tubular handling
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2. Serum potassium(3.4-4.8 mmol/l)
Hypokalemia
Vomitting, diarrhea and gastric suction, drug related
Intravenous potassium should be diluted and infused
slowly
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Hyperkalemia
Serious cardiac effects
Administration of intravenous glucose insulin,
treatment with cation exchange resin, hemodialysis Drug related: potassium sparing diuretics, ACE
inhibitors, AT antagonists and NSAIDs
Present in acute renal failure
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3. Serum calcium(2.2-2.5mmol/l or 8.8-10.2
mg/dl)
Ca corr (mg/dl)= {(4-albumin g/dl)x0.8 mg/gl} + Ca
uncorr (mg/dl)
Hypocalcemia : vit D deficiency, hypparathyroidism and
chronic renal failure
Hypercalcemia : overuse of Vit. D or thiazide diruetics.
Constipation, confusion, conjunctivits, drowsiness,
lethargy and polyuria. Most common cause is release of
calcium from bones
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Blood picture
1. Hemoglobin (13.5-17.5 g/dl for men, 12-16
g/dl for women)
Higher in polycythemia rubra vera, patients who
live at high altitude or those with chronic hypoxiclung disease
Anemia :
Types depending on MCV(80-100fl) and MCHC (33-37g/dl)
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Microcytic anemia
Low MCV and/or low MCHC
Microcytic hypochromic anemia is associated with
iron deficiency
Normocytic anemia
MCV and MCHC are within normal ranges
Associated chronic infections, rheumatoid
arthritis, hypothyroidism and malignancy, chronic
renal failure, blood loss
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Macrocytic anemia
High MCV
Liver disease, deficient dietary intake of folic acid
or vit B12
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2. White cell count (4.5-10.5 x 103/mcl)
Luecopenia (neutropenia or lymphopenia)
Elevated levels seen with infections,
corticosteroids and lithium can also increase the
count, eosinophilia (allergic reaction)
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3. Platelet count(150-450 x 103/mcl)
Surrogate marker for inflammatory reaction
Thrombocytopenia : liver disease, infections, drug
induced
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Other blood tests
Erythrocyte sedimentation rate (ESR)
Normal volume is less than 10mm/h
Westergren method
High ESR indicates increased protein level
(inflammatory condition)
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C-reactive protein
Secreted by liver in response to inflammation
Haptoglobin
Functions as an antioxidant
Helps to monitor inflammatory conditions
Coombs test
Anti-human immunoglobulin antibody and
anticomplement antibody autoimmune anemia
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Xanthochromia
Several hours after subarachnoid haemorrhage,
distinguishes from traumatic lumbar puncture
Iron, transferrin and iron binding
Transferrin total iron binding capacity
Ferritin measurement- decrease in iron deficiency
Free protporhyrinincrease in iron deficiency
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Liver function test
Albumin Present in ECF and serum
Important role in binding
Concentration depends on synthesis, volume of
distribution and rate of catabolism Synthesis- fall with increasing severity of liver disease or
malnutrition in response to inflammatory mediators
Volume of distribution increases, causes reduced serum albumin
in case of cirrhosis, fluid retention states
Increased capillary permeability results in movement of albuminfrom serum into interstitial fluid
Other causes include catabolic states- kidney, skin, intestinal wall
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Bilirubin
Elevation of bilirubin above 50micromole/l reveal
presence of jaundice.
Elevation may be due to
Increased production of bilirubin- haemolyis,
ineffective erythropoesis
Impaired transport in hepatocytes- hepatitis or drug
induced
Decreased excretion- drugs, cirrhosis, tumours
Intra-hepatic obstruction- cirrhosis, tumours
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Enzymes
Alanine transaminase
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Ammonia
Indicative of impairment of urea cycle function or
extensive hepatic damage
Hyperammonaemia >60mmol/L Valproic acid can induce encephalopathy
Amylase
Rises within 24 h of pancreatitis, then declines to
normal over 1 week
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Cardiac markers Troponins
Cardiac troponin I(cTn I) and Cardiac troponin
T(cTn T)
Creatine kinase
Increased during shock, myocardial infarction and
muscular dystrophies
Drug induced
Two protein subunits M and B
Lactate dehydrogenase
LD-1-LD-5, increase levels in damage to liver,
skeletal muscle and kidneys, anemia
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Tumor markers
Prostate specific antigen
Human chorionic gonadotropin
Carcinoembyronic antigen
Cancer antigen (CA125 and CA 19)
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Immunoglobulins
Detected by electrophoresis as bands in , ,
regions
Increased levels found in infections chronic
liver disease, autoimmune disease
Lymphatic leukaemia, lymphoma and multiple
myeloma
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Lab Test-Drug Interaction:
Pharmacological Mechanisms
Pharmacologically, a drug may cause a true change in
the concentration of the particular substance being
measured.
Allopurinol, a xanthine oxidase inhibitor, is used toreduce plasma uric acid levels in selected patients.
Incidental pharmacologic effects, other than those
primarily intended, may also occur.
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Antiovulatory drugs, used for contraceptive
purposes, cause an increase in total serum thyroxine-
binding globulin.
This results in an increase in both total serumthyroxine and unsaturated thyroxine-binding
globulin, but no significant change in unbound
(free) thyroxine.
Recognition of these effects is essential in the
interpretation of thyroid function tests.
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Drugs may also cause undesirable toxic or side
effects, as evidenced by changes in liver or renal
function tests.
Thus, in some individuals, phenothiazines willproduce a picture of obstructive jaundice, with
elevation in serum bilirubin and alkaline
phosphatase.
Knowledge of the pharmacologic effects of drugs
helps to explain a number of correct but otherwise
unexpected laboratory results.
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Lab Test-Drug Interaction:
Chemical Mechanisms
Chemically, a drug or its metabolite may interfere
with the accurate determination of the desired
constituent.
Methyldopa, used in the treatment of hypertension,will produce a false positive result for urinary
catecholamines, since it reacts in the method to
produce a green fluorescence similar to that
produced by norepinephrine.
This could lead to an erroneous diagnosis of
pheochromocytoma.
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Radiographic contrast media, containing iodinated
organic compounds, invalidate protein-bound iodine
determinations because of their own high iodine
content. Other drugs can interfere by producing extraneous
color reactions, by chelation, or by inhibition of
enzyme activity.
In uremia, associated high levels of creatinine and
uric acid can result in falsely elevated values for
glucose as measured by alkaline ferricyanide
methods.