Which statement about sensitivity, specificity, PPV, and NPV is correct?

• A. Sensitivity equals 1 - Specificity.
• B. PPV equals NPV.
• C. Sensitivity is the true positive rate; Specificity is the true negative rate; PPV is the probability a positive result is truly positive; NPV is the probability a negative result is truly negative.
• D. Sensitivity measures false positives.

Answer: (C)

Key idea: what each diagnostic metric actually measures on a disease vs test outcome table. Sensitivity is the true positive rate: TP divided by (TP plus FN). Specificity is the true negative rate: TN divided by (TN plus FP). PPV is the probability that a positive result is truly positive: TP divided by (TP plus FP). NPV is the probability that a negative result is truly negative: TN divided by (TN plus FN). This aligns with the statement that sensitivity is the true positive rate, specificity is the true negative rate, PPV is the probability a positive result is truly positive, and NPV is the probability a negative result is truly negative. A common pitfall is thinking sensitivity equals 1 minus specificity (that’s the false positive rate, not a measure of sensitivity). PPV and NPV are not generally equal; they depend on disease prevalence in the population. Also, sensitivity and specificity are intrinsic test properties, while PPV and NPV reflect how common the disease is among those tested.