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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