PLAB 1 (GMC, international medical graduates) · Statistics and evidence-based practice

Diagnostic tests and screening

8 practice questions, each with the answer and an explanation.

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  1. A new test for a condition was evaluated in 300 people. Of 100 who had the disease, 90 tested positive. Of 200 who did not, 30 tested positive. What is the sensitivity of the test?
  2. A test is evaluated in 300 people. Of 100 with the disease, 90 test positive; of 200 without it, 30 test positive. What is the specificity?
  3. A test is evaluated in 300 people. Of 100 with the disease, 90 test positive; of 200 without it, 30 test positive. What is the positive predictive value?
  4. A screening test has sensitivity 90 percent and specificity 95 percent. It is applied to 10 000 people in a population where 1 percent have the disease. Approximately what is the positive predictive v…
  5. What happens to the positive predictive value of a test if the prevalence of the disease in the tested population falls, with sensitivity and specificity unchanged?
  6. A test has sensitivity 90 percent and specificity 85 percent. What is its positive likelihood ratio?
  7. A patient has a pre-test probability of disease of 20 percent. A test with a positive likelihood ratio of 6 is positive. What is the approximate post-test probability?
  8. An NHS screening programme finds that people diagnosed through screening live longer after diagnosis than those diagnosed with symptoms, but overall death rates from the disease are unchanged. Which b…

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