Iceberg Phenomenon of Diseases

3. Iceberg Phenomenon of Diseases

4. SCREENING FOR DISEASES

5. Screening Definition: “The search for unrecognized disease or defect by means of rapidly applied tests, examinations or other procedures in apparently healthy individuals”.

6. Screening differs from periodic health examinations in-- • Capable of wide application • Relatively inexpensive • Require little physician time.

7. Concept of “Lead Time” OUTCOME Disease onset First possible point Final critical diagnosis Usual time of diagnosis A Screening time B Lead time

10. Aims and Objectives • Sort out from a large group of apparently healthy persons those who are “apparently abnormal”.

12. TYPES OF SCREENING Mass Screening Opportunistic Screening / Case detection Targeted Screening Multi-phasic Screening

13. Uses of screening Case detection - Prescriptive screening Control of disease – Prospective screening Research purpose Educational opportunities

14. CRITERIA FOR SCREENING DISEASE Prevalence should be high Should have recognizable latent period Natural history should be known A test to detect prior to onset of signs and symptoms Facilities to confirm the diagnosis

15. CRITERIA FOR SCREENING There should be an effective treatment. Agreed on policy concerning whom to treat as patients. Early detection and treatment should reduce morbidity and mortality. Expected benefits of early detection should exceeds the risks and costs.

16. Disease which is appropriate for screening

17. CRITERIA FOR SCREENING TEST Acceptability Repeatability A. Observer Variation -Intra observer variation -Inter observer variation B. Biological Variation C. Errors related to technical methods Validity (Accuracy)

18. ACCEPTABILITY The test should be acceptable to the people at whom it is aimed. Tests that are painful , discomforting or embarrassing are not likely to be accepted. E.g. Rectal examination

19. REPEATABILITY Synonyms = Reproducibility , reliability, precision The test must give consistent results when repeated more than once in the same individual under the same conditions.

20. Biological Variation Table showing variation in Blood Pressure over a 24 hour period LEON GORDIS

21. Intra-observer Variation LEON GORDIS

22. Inter-observer Variation LEON GORDIS

23. VALIDITY Synonym = Accuracy The extent to which the test measures what it purports to measure. The validity of a screening test is measured in terms of : • Sensitivity • Specificity • Predictive values

24. Comment on the diagram ? Test is repeatable but NOT valid Test is both repeatable and valid

25. Comment on the values ?True measure of SBP being 120 mm Hg in an individual 5 measurements PRECISION VALIDITY Valid Precise Valid Not Precise Precise Not valid Not precise Not valid

31. VALIDITY OF A TEST • Sensitivity - Ability of a test to detect who are suffering from disease (true +ves) • 2. Specificity - Ability to detect those who are not suffering from disease (true -ves) • 3.Positive predictive value – patient with a positive test result has the disease in question. • 4. Negative predictive value – patient with a negative test result do not have the disease in question

32. Sensitivity a/a+c x 100 Specificity d/b+d x 100 Predictive value of a +ve test a/a+b x 100 Predictive value of a -ve test d/c+d x 100 Percentage of False +ve b/b+d x 100 Percentage of False -ve c/a+c x 100

33. Example • Sensitivity = 40/140 x 100 = 28.57% • Specificity = 9840/9860 x 100 = 99.79% • Predictive value of a +ve test = 40/60 x 100 = 66.66% • Predictive value of a -ve test = 9840/ 9940 x100 = 98.9% • Percentage of False +ve x 100 = 20/9860 x 100 = 0.20% • Percentage of False -ve x 100 = 100/140 x 100 = 71.4%

35. Evaluation of screening programme • Randomized controlled trial • Uncontrolled trials

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