Which formula represents the Negative Predictive Value (NPV)?

• A. TN/(TN+FN)
• B. TP/(TP+FP)
• C. FP/(TN+FP)
• D. FN/(FN+TN)

Answer: (A)

Negative Predictive Value measures how trustworthy a negative test result is. It answers: among those who test negative, what fraction truly do not have the disease? This is calculated by comparing the true negatives to all negative results, because the total of negative results includes both true negatives and false negatives. Therefore, the formula is true negatives divided by (true negatives plus false negatives), or TN/(TN+FN).

This differs from the idea behind a positive test result, which relates to how many of those who test positive actually have the disease (the positive predictive value, TP/(TP+FP)). It also differs from the false positive rate, which looks at how many non-diseased individuals test positive (FP/(TN+FP)). The option FN/(FN+TN) would describe the portion of negative results that are actually false negatives, which is not the negative predictive value.