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1 Screening in Public Health Practice
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Screening in Public Health Practice
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Screening
Definition: Presumptive identification of an unrecognized disease or defect by the application of tests, examinations, or other procedures. Classifies asymptomatic people as likely or unlikely to have a disease or defect. Usually not diagnostic.
Purpose: Delay onset of symptomatic or clinical disease. Improve survival.
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Screening
Seems simple but is complex. There are hidden costs and risks. Screening can create morbidity and anxiety. Must be aware of biases.
For screening to be successful you need a:
Suitable disease Suitable test Suitable screening program
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What type of screening program is this?
Drs. Poke and Jab (1993) conducted research at all shopping malls in California to detect high blood pressure and to warn people of the potential for hypertension. Their subjects were chosen from volunteers passing by the mall. Which type of program is this?
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Suitable Disease
Has serious consequences Is progressive Disease treatment must be effective at an
earlier stage Prevalence of the detectable pre-clinical phase
must be high Examples of suitable diseases: breast cancer,
cervical cancer, hypertension
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Natural History of Disease
20 30 40 50 60 70 Years
A B C D
Biological Disease Symptoms Death Onset Detectable Develop By Screening
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Natural History of Disease
Total pre-clinical phase = A to C (Age 30 to Age 60) = 30 years
Detectable pre-clinical phase (DPCP) = B to C (Age 45 to Age 60) = 15 years
DPCP varies with the test, the disease, and the individual
Lead Time: Duration of time by which the diagnosis is advanced as a result of screening. B to C (Age 45 to Age 60) = 15 years
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Suitable Test
Ideally, it's inexpensive, easy to administer, has minimal discomfort has high level of validity and reliability
Valid Test: Does what it's supposed to do, that is, correctly classify people with pre-clinical disease as positive and people without pre-clinical disease as negative
Reliable Test: Gives you same results on repetition
Validity is more important than reliability
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Suitable Test
Yes No Total
Positive a b a + b
Negative c d c + d
Total a + c b + d a + b + c+ d
Disease Status (Truth)
Screening Test Result
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Suitable Test
Measures of test validity
Sensitivity - enables you to pick up the cases of disease
Sensitivity = a / a + c = those that test positive / all with disease
Specificity - enables you to pick out the no diseased people
Specificity- enables you to pick out the non diseased people
Specificity = d / b + d = those that test positive / all with disease
Valid test has high sensitivity and specificity
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Suitable TestBreast Cancer Screening Program of Heath Insurance Plan (HIP)Women assigned to screening or usual care. Screening consisted of yearlymammogram and physical exam. Five years of follow‑up produced these results:
Confirmed
Not Confirmed
Total
Positive 132 983 1,115
Negative 45 63,650 63,695
Total 177 64,633 64,810
Breast Cancer
ScreeningTest
Result
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Suitable Test
Sensitivity = 132/177 = 74.6% Specificity = 63,650/64,633 = 98.5% Interpretation: The screening was very good at picking
out the women who did not have cancer (see specificity) but it missed 25% of the women who did have cancer (see sensitivity).
To measure sensitivity and specificity you can wait for disease to develop (as above) or you can measure the results of the screening test against the outcome of another screening or diagnostic test (the Gold Standard).
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Suitable Test
Criterion of Positivity ‑ test value at which the screening test outcome is considered positive
Test Result
Clearly Negative Grey Zone Clearly Positive
-------------------------??????????????????---------------------
A B C
Criterion of positivity affects sensitivity and specificity. Must trade off between the two.
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Suitable Test
What are the sensitivity and specificity if A (or B or C) is used as the cutoff for a positive result?
If criterion is low (Point A) then sensitivity is good but specificity suffers. If criterion is high (Point C) then specificity is good but sensitivity suffers.
Decisions about criterion of positivity involves weighing the cost of false positives against the cost of false negatives.
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Suitable Screening Program
Definition of a screening program: Application of a specific test in a specific population for a specific disease
You want to determine if screening program is successful. Does it reduce morbidity and mortality? How to evaluate?
Feasibility Measures Effectiveness Measures
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Evaluation of Screening Program
1. Feasibility Measures Acceptability, cost, predictive value of a positive test (PV+),
predictive value of a negative test (PV-) Disease
Yes
Status
No
Total
Screening Test
Positive a b a + b
Result Negative c d c + d
Total a + c b + d a + b + c + d
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Evaluation of Screening Program
Predictive value of a positive test (PV+)
=
A/aa a / a + b
=
Number who test positive with disease / Number with positive result
Predictive value of a negative test (PV-)
=
d / c + d
=
Number who test negative without disease / Number with negative result
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Evaluation of Screening Program
Breast Cancer Screening Program of HIP
CancerConfirmed
Cancer NotConfirmed
Total
Positive 132 983 1,115
Negative 45 63,650 63,695
Total 177 64,633 64,810
Breast Cancer
Screening Test
PV+ = 132/1115 = 11.8%PV- = 63,650/63,695 = 99.9%
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Evaluation of Screening Program
PV will increase when sensitivity, specificity, and disease prevalence increases.
For example, PV+ will increase if you perform breast cancer screening on higher risk population (i.e. women with a family history of breast cancer)
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Evaluation of Screening Program
Demonstration of how prevalence effects PV Use screening test with 99.9% sensitivity and 99.9% specificity
in two populations:
Population A) Two positive results. One will be true positive. One will be a test
error. PV+ is 50%
: 1,000 people with low prevalence of disease (1/1,000 Population B: 1,000 people with high prevalence of disease
(10/1,000)
Eleven positive test results. 10 will be true positives. One will be a test error. PV+ is 10/11 or 90.9%
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Summary of Screening
Screening is the presumptive identification of unrecognized disease by the application of tests, exams, etc.
Suitable disease must be serious with important consequences and progressive
Suitable test must have low cost, be acceptable, and have a high degree of validity
Validity is measured by sensitivity and specificity
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A new screening test for Lyme disease is developed for use in the general population. The sensitivity and specificity of the new test are 60% and 70%, respectively. Three hundred people are screened at a clinic during the first year the new test is implemented. (Assume the true prevalence of Lyme disease among clinic attendees is 10%.) What are the number of false positives?
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What are the number of false positives?
Disease
Yes
Status
No
Total
Screening Test
Positive a b a + b
Result Negative c d c + d
Total a + c b + d a + b + c + d
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Disease
Yes
Status
No
Total
Screening Test
Positive 18 81 99
Result Negative 12 189 201
Total 30 270 300
What are the number of false positives?
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Summary of Screening
Screening programs administer screening tests in particular populations
Programs are evaluated mainly by examining predictive value and outcome measures such as stage distribution and cause-specific mortality
Evaluation must consider lead-time bias, length-biased sampling, and volunteer bias.
