Epidemiology – Positive and Negative Predictive Value (PPV + NPV)

Strongly related to the concepts of sensitivity and specificity are the concepts of PPV and NPV.  These terms are quite similar (and can be confused) so it’s important to remember that sensitivity and specificity measure the accuracy of the test (not any relation to the disease or population), whereas PPV and NPV measure the proportion of people whose test results reflect their health status and therefore *are* affected by the disease prevalence… it’s a sublte, but important distinction! (Especially to diffrentiate what is being asked of you in an exam…)

Definition – Positive Predictive Value (PPV)

  • PPV = the proportion of individuals who test positively (a+b) AND trully have the disease (a)
  • Formulae = a / (a+b)
  • *Importnat* PPV increases with high prevalence of disease

Definition – Negative Predictive Value (NPV)

  • NPV = the proportion of individuals who test negatively (c+d) AND trully do not have the disease (d)
  • Formulae = d / (c+d)
  • *Important* NPV descreases with high prevalence of disease
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