What is Screening

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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
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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
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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?)
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Screening Test Concerned with a Functional
Definition of Normality versus Abnormality
Screening Test
Abnormal
Normal
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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

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• 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)

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Screening for Glaucoma using IOP
True Cases of Glaucoma
Yes No IOP gt 22 Yes 50 100 No
50 1900 (total) 100 2000
Sensitivity 50 (50/100) False
Negative50 Specificity 95 (1900/2000)
False Positive5
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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
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Reliability (reproducibility)
Agreement within and between
examiners ________________________________________
________
Inter-Observer Agreement in Grading Severity of
Cataract
Examiner 1 Grade
Examiner lt1 1-lt2 2-lt3 3-lt4 4
2 lt1 10 2 1 0
0 1-lt2 1 20 2 0 0 2-lt3 0
1 20 1 0 3-lt4 0 0
1 10 2 4 0 0 0 2 5
Agreement 81.3 Kappa 0.76
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Validity versus Reliability of Screening Test
Examiner 1 Examiner 2 Examiner 3
Good Reliability
Low Validity
True cases
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Yield from a Screening Test for Disease
X Predictive Value
Screening Test
X
X
X
X
X
X
Positives
Negatives
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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

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Screening for Glaucoma using IOP
True Cases of Glaucoma
Yes No IOP gt 22 Yes 50 100 No
50 1900 (total) 100 2000
Specificity 95 (1900/2000) False
Positive5 Positive Predictive Value 33
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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 gt90 sensitivity, 99.9 specificity -Screen
for nearsighted children proposes 80
sensitivity, gt95 specificity -Pre-natal genetic
questionnaire could be 99 sensitive, 80
specific
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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