Which statement correctly defines sensitivity and specificity?

• A. Sensitivity equals true negatives divided by all actual negatives.
• B. Specificity equals true positives divided by all actual positives.
• C. Sensitivity equals true positives divided by all actual positives.
• D. Specificity equals true negatives divided by all actual positives.

Answer: (C)

Sensitivity measures how well a test detects people who actually have the condition. It is the proportion of true positives among all those who truly have the disease, calculated as true positives divided by (true positives plus false negatives). That makes the statement correct: sensitivity = TP / (TP + FN).

In contrast, specificity tells how well the test identifies people who do not have the condition, calculated as true negatives divided by (true negatives plus false positives). So the common reference values are TP/(TP+FN) for sensitivity and TN/(TN+FP) for specificity.

To understand why the other formulations are not correct: sensitivity is not true negatives over all actual negatives—that would be a measure of specificity. Likewise, specificity is not true positives over all actual positives, since that would mix up who is being considered (positives vs. negatives) and uses the wrong denominator. And again, specificity is not true negatives over all actual positives for the same reason.