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 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?)
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)
Screening for Glaucoma using IOP
True Cases of Glaucoma
IOP > 22:
Yes
No
Yes
50
100
No
50
1900
(total)
100
2000
Sensitivity = 50% (50/100)
False Negative=50%
Specificity = 95% (1900/2000) False Positive=5%
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
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
Validity versus Reliability of Screening Test
Examiner 1
Examiner 2
Examiner 3
Good Reliability
True cases
Low Validity
Yield from a Screening Test for Disease X
Predictive Value
Screening Test
X
X
X
X
X
X
Negatives
Positives
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
Screening for Glaucoma using IOP
True Cases of Glaucoma
IOP > 22:
Yes
No
Yes
50
100
No
50
1900
(total)
100
2000
Specificity = 95% (1900/2000)
Positive Predictive Value =33%
False Positive=5%
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 problem
2. There should be a recognizable early or latent stage
3. There should be an accepted treatment for persons with
condition
4. The screening test is valid, reliable, with acceptable yield
5. The test should be acceptable to the population to be
screened
6. The cost of screening and case finding should be
economically balanced in relation to medical care as a whole