Sensitivity and specificity are stable properties of screening tests and, as a result, are unaffected by the prevalence of a

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 “Sensitivity and specificity are stable properties of screening tests and, as a result, are unaffected by the prevalence of a disease. Predictive value, however, is very much affected by the prevalence of the condition being screened. Many screening tests are validated upon groups that have a contrived prevalence of disease (e.g., approximately 50%). This prevalence would usually be higher than what is found in clinical practice” (Friis & Sellers, 2014, p. 478). 

• Explain the paradox of declining mortality rates for a disease given constant case fatality rates. For a specific case, use Rabies.
• What is the relationship between reliability and validity? Is it possible for a measure to be reliable and invalid? Conversely, is it possible for a measure to be unreliable and valid? What measures of validity are used to evaluate screening tests? What are their formulas?
• Assume that the fasting blood level of a lipid is normally distributed in the population of people who do not have disease “X.” There is a smaller distribution curve of the fasting blood levels of this lipid, which also is normal in shape, for the population of persons who have disease “X,” and the curve overlaps the upper end (right side) of the curve for people without the disease. Draw distribution curves for the diseased and non-diseased populations and discuss the effects upon sensitivity and specificity of setting the cut point for disease and non-disease at various positions on the two overlapping curves.
• How does the predictive value of a screening test vary according to the prevalence of disease?
• A serologic test is being devised to detect a hypothetical chronic disease. Three hundred individuals were referred to a laboratory for testing. One hundred diagnosed cases were among the 300. A serologic test yielded 200 positives, of which one-fourth were true positives. Calculate the sensitivity, specificity, and predictive value of this test. (Hint: After setting up the appropriate 2 by 2 table, find missing data by subtraction. The numbers for the cells should then correspond to the numbers shown in Appendix 11., ., p. 734)
• The prevalence of undetected diabetes in a population to be screened is approximately 1.5%, and it is assumed that 10,000 persons will be screened. The screening test will measure blood serum glucose content. A value of 180 mg% or higher is considered positive. The sensitivity and specificity associated with this screening test are 22.9% and 99.8%, respectively.
  • What is the predictive value of a positive test?
  • What is the predictive value of a negative test?

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