Positive predictive value

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Positive predictive value of a test/investigation is defined as the proportion of patients with positive results being truly diseased.

Calculation

Positive predictive value = true positives (TP) detected / total positive results

(total positive results = true positive (TP) + false positive (FP)

Bayes' theorem

One can also determine the PPV with an estimate of sensitivity, specificity, and pretest probability (p).

PPV = [(sensitivity) x (p)] / [sensitivity x (p) + (1 - specificity) x (1 - p)]

Practical points

  • unlike sensitivity and specificity, PPV is highly dependent on the prevalence of the disease in the target population