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Screening


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Definition

The presumptive identification of unrecognized disease

or defect by the application of tests, examination or

other procedures which can be applied rapidly.

Screen ng sort out apparent y we persons w o

probably have a disease from those probably do not.


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Main Aim of Screening

To reverse, halt, or slow the progression of disease

more effectively than would probably normally happen.

To alter the natural course of disease for a better

outcome or n v ua s a ecte .

Reduce morbidity and mortality through early detection and treatment


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h r Aim f r nin

Protect society from contagious disease

Rational allocation of resources

e ec on o ea y n v ua s:

employment, military

Research; study on natural history of


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Two-by-Two Table for Screening Evaluation

Measures of validity: sensitivity and specificity


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Sensitivity of a Screening Test

Sensitivity : the proportion of people with a disease

who have a positive test for the disease

a/a+c


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Specificity of a Screening Test

Specificity is the proportion of people without the

disease who have a negative test

d/b+d


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Positive Predictive Value

Positive predictive value: is the probability of the

presence of the disease in a person with a positive(abnormal) test result.

(+PV = a/a+b)


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Negative Predictive Value

Negative predictive value is the probability of not

having the disease when the test result is negative(normal).

(-PV = d/c+d)


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-Disease serious & High risk with further

Disease is highly

communicable

agnost c tec n ques

High cost with further

Subsequent evaluation of

positives is not costly and

No definitive treatment

exist

has minimal risk

Sensitivity Specificity

Sensitivity and specificity are affected by the cut-off value of measure at

which a test is defined as positive.


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Predictive Value Positive (Yield)

The yield of a test result is affected by:

Specificity of the test

Prevalence of the disease


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,specificity and prevalence on yield


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Changing Sensitivity


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Changing Specificity


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Changing Prevalence


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Strategies to Increase Yield

Select a test with high specificity Hi h sensitivit > Low false - > Hi h PV-

High specificity > Low false + > High PV+

Select disease with hi h revalence of re-clinical stage


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Success Factors for Screening Programs

The health problem should be important enough to be worth detecting.

Availability of an acceptable intervention which is effective.

Presence of a recognizable latent or early "asymptomatic" stage.

Presence of a suitable and acceptable test.

The natural history of the condition should be adequately understood.

Presence of policy regarding when the intervention is appropriate.

The cost of detecting the problem and its remedy should be reasonable.

" ", - .


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Potential Source of Bias in Screening

Self-selection (volunteer) bias

Lea t me as ear y agnos s


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-


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Length Bias in Screening


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Study Design for Evaluation of Screening

Ideally experimental

Most commonly used is cohort

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