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Screening in Public Health Practice


Title: EB711 Lecture 13 Author: Boston University BUMC Last modified by: Cookseyax Created Date: 4/6/2001 2:36:13 PM Document presentation format – PowerPoint PPT presentation

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Title: Screening in Public Health Practice

Screening in Public Health Practice
  • 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
  • Purpose Delay onset of symptomatic or clinical
    disease. Improve survival.

  • 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

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?

Suitable Disease
  • Has serious consequences
  • Is progressive
  • Disease treatment must be effective at an earlier
  • Prevalence of the detectable pre-clinical phase
    must be high
  • Examples of suitable diseases breast cancer,
    cervical cancer, hypertension

Natural History of Disease
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
  • 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

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
  • Validity is more important than reliability

Suitable Test
Disease Status (Truth)
Screening Test Result
Suitable Test
  • Measures of test validity
  • Sensitivity - enables you to pick up the cases of
  • 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

Suitable Test
  • Breast Cancer Screening Program of Heath
    Insurance Plan (HIP)
  • Women assigned to screening or usual care.
    Screening consisted of yearly
  • mammogram and physical exam. Five years of
    follow-up produced these results

Breast Cancer
Screening Test Result
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).

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.

Suitable Test
  • What are the sensitivity and specificity if A (or
    B or C) is used as the cutoff for a positive
  • 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.

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

Evaluation of Screening Program
  • 1. Feasibility Measures
  • Acceptability, cost, predictive value of a
    positive test (PV), predictive value of a
    negative test (PV-)

Evaluation of Screening Program
Evaluation of Screening Program
  • Breast Cancer Screening Program of HIP

Breast Cancer
Screening Test
PV 132/1115 11.8 PV- 63,650/63,695 99.9
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

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
  • 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

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

  • 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?

What are the number of false positives?
What are the number of false positives?
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.