Question 15

Define the concepts of:

  • Sensitivity (20% of marks)
  • Specificity (20% of marks)
  • Positive predictive value (PPV) (20% of marks)
  • Negative predictive value (NPV) (20% of marks)

Explain how the prevalence of a disease influences the PPV and the NPV of a
diagnostic test (20% of marks).

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College Answer

The question required a precise definition of each term. Better answers included a 2
x 2 table denoting true and false positives and negatives. Discussion of sensitivity,
specificity, positive predictive value (PPV) and negative predictive value (NPV)
needed to be consistent with accurately quoted formulae. The directional influence
of prevalence on PPV and NPV should have been stated clearly. Bayes’ Theorem
was noted in more comprehensive answers.
Syllabus: Evidence Based Medicine 2f
Recommended sources: Statistical methods for anaesthesia and intensive care,
Myles and Ginn.


  • Sensitivity = true positives / (true positives + false negatives)
    • This is the proportion of disease which was correctly identified
  • Specificity = true negatives / (true negatives + false positives)
    • This is the proportion of healthy patients in who disease was correctly excluded
  • Positive Predictive Value = true positives / total positives (true and false)
    • The proportion of the positive tests results which are actually positive.
  • Negative Predictive Value = true negatives / total negatives (true and false)
    • The proportion of negative test results which are actually negative.