What is Screening?

Basic Public Health Concepts

Sheila West, Ph.D.El Maghraby Professor of Ophthalmology

Wilmer Eye Institute Johns Hopkins University

SCREENING: DEFINITION

“The PRESUMPTIVE identification of UNRECOGNIZED disease or defect by the application of tests, exams or other procedures which can be applied RAPIDLY to sort out apparently well persons who PROBABLY have a disease from those who PROBABLY do not”*

Key Elements: disease/disorder/defect

screening test

population

*Commission on Chronic Illness, 1957

Issues in Screening

Disease

-Disease/disorder should be an important public health problem High prevalenceSerious outcome

-Early Detection in asymptomatic (pre-clinical) individuals is possible

-Early detection and treatment can affect the course of disease (or affect the public health problem?)

Screening Test Concerned with a Functional Definition of

Normality versus Abnormality

Screening Test

Normal Abnormal

Criteria for Evaluating a Screening Test

•Validity: provide a good indication of who does and does not have disease

-Sensitivity of the test

-Specificity of the test

•Reliability: (precision): gives consistent results when given to same person under the same conditions

•Yield: Amount of disease detected in the population, relative to the effort

-Prevalence of disease/predictive value

Validity of Screening Test (Accuracy)

- Sensitivity: Is the test detecting true cases of disease? (Ideal is 100%: 100% of cases are detected)

-Specificity: Is the test excluding those without disease? (Ideal is 100%: 100% of non-cases are negative)

True Cases of Glaucoma

Yes No

IOP > 22: Yes 50 100

No 50 1900

(total) 100 2000

Sensitivity = 50% (50/100) False Negative=50%Specificity = 95% (1900/2000) False Positive=5%

Screening for Glaucoma using IOP

Consider:

-The impact of high number of false positives: anxiety, cost of further testing

-Importance of not missing a case: seriousness of disease, likelihood of re-screening

Where do we set the cut-off for a screening test?

Reliability (reproducibility)

Agreement within and between examiners________________________________________________

Inter-Observer Agreement in Grading Severity of Cataract

Examiner <1 1-<2 2-<3 3-<4 4 2

<1 10 2 1 0 0

1-<2 1 20 2 0 0

2-<3 0 1 20 1 0

3-<4 0 0 1 10 2

4 0 0 0 2 5

% Agreement = 81.3%Kappa = 0.76

Examiner 1: Grade

Validity versus Reliability of Screening Test

Examiner 1 Examiner 2 Examiner 3

True cases

Good Reliability

Low Validity

Yield from a Screening Test for Disease XPredictive Value

X

X

Screening Test

Negatives Positives

X

X

X

X

Yield from the Screening Test: Predictive Value

•Relationship between Sensitivity, Specificity, and Prevalence of Disease

Prevalence is low, even a highly specific test will give large numbers of False Positives

•Predictive Value of a Positive Test (PPV): Likelihood that a person with a positive test has the disease

•Predictive Value of a Negative Test (NPV): Likelihood that a person with a negative test does not have the disease

True Cases of Glaucoma

Yes No

IOP > 22: Yes 50 100

No 50 1900

(total) 100 2000

Specificity = 95% (1900/2000) False Positive=5%Positive Predictive Value =33%

Screening for Glaucoma using IOP

How Good does a Screening Test have to be?

IT DEPENDS

-Seriousness of disease, consequences of high false positivity rate:

-Rapid HIV test should have >90% sensitivity, 99.9% specificity

-Screen for nearsighted children proposes 80% sensitivity, >95% specificity

-Pre-natal genetic questionnaire could be 99% sensitive, 80% specific

Principles for Screening Programs

1. Condition should be an important health problem2. There should be a recognizable early or latent stage3. There should be an accepted treatment for persons with

condition4. The screening test is valid, reliable, with acceptable yield5. The test should be acceptable to the population to be

screened6. The cost of screening and case finding should be

economically balanced in relation to medical care as a whole