SCREENINGSCREENING

The search for unrecognized The search for unrecognized disease or defect by means of disease or defect by means of rapidly applied tests, rapidly applied tests, examinations or other examinations or other procedures in apparently healthy procedures in apparently healthy individuals individuals

SCREENING (Conti)SCREENING (Conti)

Testing for infection or disease in Testing for infection or disease in population or individuals who are not population or individuals who are not seeking health careseeking health care

Active search for disease among Active search for disease among apparently healthy people apparently healthy people

SCREENING (Conti)SCREENING (Conti)

The active search for disease among apparently The active search for disease among apparently healthy people is a fundamental aspect of healthy people is a fundamental aspect of prevention prevention

To bring such examination within the reach of To bring such examination within the reach of large masses of people with minimal large masses of people with minimal expenditure of time and moneyexpenditure of time and money

Conserve physician timeConserve physician time Administration of simple, inexpensive lab. TestsAdministration of simple, inexpensive lab. Tests Preventive care functionPreventive care function

SCREENING (Conti)SCREENING (Conti)

Screening test is not intended to be Screening test is not intended to be a diagnostic test. It is only an initial a diagnostic test. It is only an initial examination. Those who are found examination. Those who are found to have positive test results are to have positive test results are referred to a physician for further referred to a physician for further diagnosis and treatmentdiagnosis and treatment..

AIMS AND OBJECTIVESAIMS AND OBJECTIVES

To sort out from a large group of apparently To sort out from a large group of apparently healthy persons who likely to have the diseasehealthy persons who likely to have the disease

To sort out from a large group of apparently To sort out from a large group of apparently healthy persons who are at increased risk of healthy persons who are at increased risk of disease under studydisease under study

To bring those who are apparently abnormal To bring those who are apparently abnormal under medical supervision and treatment.under medical supervision and treatment. Earlier diagnosis and subsequent favourably alters Earlier diagnosis and subsequent favourably alters

the natural history of the disease in a significant the natural history of the disease in a significant proportion of those who are identified as positive. proportion of those who are identified as positive.

Screening Screening

Screening testScreening test Diagnostic testDiagnostic test

Done on apparently healthyDone on apparently healthy Done on those who are sickDone on those who are sick

Applied to groupsApplied to groups Applied to single patientApplied to single patient

Results are finalResults are final Diagnosis is not finalDiagnosis is not final

Based on one criteriaBased on one criteria Based on evaluation of S/SBased on evaluation of S/S

Less accurate, expensiveLess accurate, expensive More accurate, expensiveMore accurate, expensive

Not basis for treatmentNot basis for treatment Basis for treatmentBasis for treatment

Initiative comes from Initiative comes from investigatorinvestigator

Initiative comes from a pt.Initiative comes from a pt.

USES OF SCREENINGUSES OF SCREENING

Case detectionCase detection Control of diseaseControl of disease Research purposeResearch purpose Educational opportunitiesEducational opportunities

TYPES OF SCREENINGTYPES OF SCREENING

MASS SCREENINGMASS SCREENING HIGH RISK OR SELECTIVE HIGH RISK OR SELECTIVE

SCREENINGSCREENING MULTIPHASIC SCREENINGMULTIPHASIC SCREENING

Criteria for screening Criteria for screening

DISEASE:DISEASE: Important Health problem-high prevalence rateImportant Health problem-high prevalence rate Recognizable latent periodRecognizable latent period Test that detects disease prior to the onset of signs Test that detects disease prior to the onset of signs

and symptomsand symptoms Facilities must be available to confirm the diagnosisFacilities must be available to confirm the diagnosis There is an effective treatmentThere is an effective treatment Good evidence that early detection and treatment Good evidence that early detection and treatment

reduces morbidity and mortalityreduces morbidity and mortality Expected benefits must exceed risk and cost.Expected benefits must exceed risk and cost.

SCREENING TESTSCREENING TEST

AcceptabilityAcceptability Repeatability Repeatability Validity Validity Yield Yield SimplicitySimplicity Safety Safety Rapidity Rapidity Ease of administration Ease of administration Cost Cost

Repeatability Repeatability

Reliability, precision, reproducibilityReliability, precision, reproducibility Test must give consistent result when Test must give consistent result when

repeated more than once on the same repeated more than once on the same individual or material under the same individual or material under the same conditions. conditions.

Repeatability depends upon three major Repeatability depends upon three major factors:factors:

Repeatability (Conti)Repeatability (Conti)

A). A). Observer variationObserver variation Intra observer variationIntra observer variation Inter observer variation Inter observer variation

B). Biological (subject) variationB). Biological (subject) variation

C). Errors relating to technical methodsC). Errors relating to technical methods

VALIDITYVALIDITY

What extent the test accurately measures What extent the test accurately measures which it purports to measure.which it purports to measure.

It expresses the ability of a test to It expresses the ability of a test to separate or distinguish those who have a separate or distinguish those who have a disease from those who do not.disease from those who do not.

Validity has two components:Validity has two components: SensitivitySensitivity Specificity Specificity

Evaluation of screening testEvaluation of screening test

SCREENING SCREENING TEST TEST

RESULTRESULT

DIAGNOSIS DIAGNOSIS

TOTAL TOTAL

DISEASEDDISEASED NOT DISEASED NOT DISEASED

PositivePositive true positive (a)true positive (a) false positive (b)false positive (b) A + bA + b

NegativeNegative false negative (c) false negative (c) true negative (d)true negative (d) C + dC + d

TotalTotal a+ca+c b + db + d A+ b+ c+ dA+ b+ c+ d

Validity (Conti)Validity (Conti)

SensitivitySensitivity The ability of a test to identify correctly all The ability of a test to identify correctly all

those who have the disease = true positivethose who have the disease = true positive Sensitivity= a / a + c X 100Sensitivity= a / a + c X 100 90% sensitivity means that 90% of diseased 90% sensitivity means that 90% of diseased

people screened by test will give true people screened by test will give true positive result and remaining 10% false positive result and remaining 10% false negative result.negative result.

Validity (Conti)Validity (Conti)

SpecificitySpecificity:: The ability of a test to identify correctly all those who The ability of a test to identify correctly all those who

do not have the disease = true negativedo not have the disease = true negative specificity= d / b + d X 100specificity= d / b + d X 100 90% specificity means that 90% of non diseased 90% specificity means that 90% of non diseased

people screened by test will give true negative result people screened by test will give true negative result and remaining 10% of non diseased people and remaining 10% of non diseased people screened by the test will be wrongly classified as screened by the test will be wrongly classified as diseased when they are not.diseased when they are not.

False negativeFalse negative

It means that patients who actually have It means that patients who actually have the disease are told that they do not have the disease are told that they do not have the disease.the disease.

A screening test which is very sensitive A screening test which is very sensitive has few false negatives. has few false negatives.

The lower the sensitivity, the larger will The lower the sensitivity, the larger will be the number of false negatives.be the number of false negatives.

False positiveFalse positive

That the patients do not have the disease That the patients do not have the disease are told that they have the disease in are told that they have the disease in question.question.

Normal healthy people will be subjected Normal healthy people will be subjected to further diagnostic tests and will cause to further diagnostic tests and will cause anxiety, inconvenience, discomfort and anxiety, inconvenience, discomfort and expense----until freedom from the expense----until freedom from the disease is established.disease is established.

Validity Validity

Predictive value of a test:Predictive value of a test: It reflects the diagnostic power of the test. It reflects the diagnostic power of the test. It measures the performance of a screening testIt measures the performance of a screening test

Predictive value of a positive test indicates that Predictive value of a positive test indicates that probability that a patient with a positive test probability that a patient with a positive test result has, in fact, the disease in question.result has, in fact, the disease in question. More prevalent disease----more predictive value of a More prevalent disease----more predictive value of a

positive screening testpositive screening test

Evaluation of screening testEvaluation of screening test

SCREENING SCREENING TEST RESULTTEST RESULT

DIAGNOSIS DIAGNOSIS

TOTAL TOTAL

DISEASEDDISEASED NOT DISEASED NOT DISEASED

PositivePositive true positive (a)=40true positive (a)=40 false positive (b)=20false positive (b)=20 A + b=60A + b=60

NegativeNegative false negative false negative (c)=100 (c)=100 true negative (d)=9840true negative (d)=9840 C + d=9940C + d=9940

TotalTotal a+c=140a+c=140 b + d=9860b + d=9860 A+ b+ c+ dA+ b+ c+ d=10000=10000

Validity Validity Sensitivity =40/140X100=28.57%Sensitivity =40/140X100=28.57%

True positiveTrue positive Specificity = 9840/9860X100=99.79%Specificity = 9840/9860X100=99.79%

True negativeTrue negative False negative=100/140X100=71.4%False negative=100/140X100=71.4% False positive=20/9860X100=0.20%False positive=20/9860X100=0.20% Predictive value of a positive test= a / ( a + b) Predictive value of a positive test= a / ( a + b)

=40/60X100=66.65%=40/60X100=66.65% Predictive value of a negative test = Predictive value of a negative test =

9840/9940X100=98.9%9840/9940X100=98.9%