If you change the cutoff for defining disease, what happens to sensitivity and specificity?

• A. They stay the same regardless of cutoff.
• B. Only sensitivity changes with cutoff.
• C. Only specificity changes with cutoff.
• D. Both sensitivity and specificity change when the cutoff is changed.

Answer: (D)

The main idea being tested is how changing the cutoff for a positive result shifts the test’s sensitivity and specificity. Sensitivity is the fraction of actual diseased individuals correctly identified, and specificity is the fraction of non-diseased individuals correctly identified. When you lower the threshold for calling someone diseased, you catch more of the true positives, so sensitivity increases, but you also misclassify more healthy people as diseased, so specificity decreases. If you raise the threshold, you miss more diseased cases (lower sensitivity) but correctly exclude more non-diseased people (higher specificity). Because the cutoff affects who is labeled positive and who is labeled negative, both sensitivity and specificity change with the cutoff.