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What is Screening? Basic Public Health Concepts Sheila West, Ph.D.

What is Screening? Basic Public Health Concepts Sheila West, Ph.D. El Maghraby Professor of Ophthalmology Wilmer Eye Institute Johns Hopkins University. SCREENING: DEFINITION

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What is Screening? Basic Public Health Concepts Sheila West, Ph.D.

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  1. What is Screening? Basic Public Health Concepts Sheila West, Ph.D. El Maghraby Professor of Ophthalmology Wilmer Eye Institute Johns Hopkins University

  2. 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

  3. Issues in Screening Disease -Disease/disorder should be an important public health problem High prevalence Serious 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?)

  4. Screening Test Concerned with a Functional Definition of Normality versus Abnormality Screening Test Abnormal Normal

  5. 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

  6. 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)

  7. Screening for Glaucoma using IOP 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%

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

  9. Reliability (reproducibility) Agreement within and between examiners ________________________________________________ Inter-Observer Agreement in Grading Severity of Cataract Examiner 1: Grade 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

  10. Validity versus Reliability of Screening Test Examiner 1 Examiner 2 Examiner 3 Good Reliability Low Validity True cases

  11. Yield from a Screening Test for Disease X Predictive Value Screening Test X X X X X X Positives Negatives

  12. 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

  13. Screening for Glaucoma using IOP 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%

  14. 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

  15. Principles for Screening Programs • Condition should be an important health problem • There should be a recognizable early or latent stage • There should be an accepted treatment for persons with condition • The screening test is valid, reliable, with acceptable yield • The test should be acceptable to the population to be screened • The cost of screening and case finding should be economically balanced in relation to medical care as a whole

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