Screening in Public Health Practice

1. Screening in Public Health Practice

2. 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.

3. 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 • Suitablescreening program

4. 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?

5. 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

6. Natural History of Disease

7. 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

8. 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

9. Suitable Test Disease Status (Truth) Screening Test Result

10. 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

11. 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

12. 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).

13. 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.

14. 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.

15. 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

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

17. Evaluation of Screening Program

18. 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%

19. 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)

20. 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%

21. 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

22. 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?

23. What are the number of false positives?

24. What are the number of false positives?

25. 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.