American Board of Internal Medicine (ABIM) Certification Practice Exam

Which test is NOT among the confirmatory tests for Cushing syndrome?

1. 24-hour urine free cortisol testing

2. 1-mg dexamethasone suppression test

3. Evening salivary cortisol measurement

4. Blood glucose tolerance test

The correct answer is: Blood glucose tolerance test

The blood glucose tolerance test is not considered a confirmatory test for Cushing syndrome. Cushing syndrome is characterized by excessive cortisol levels, and confirmatory tests are specifically aimed at measuring cortisol production or response. The 24-hour urine free cortisol test directly quantifies cortisol excretion over a full day, which is highly indicative of hypercortisolism when elevated. The 1-mg dexamethasone suppression test assesses how cortisol levels react to dexamethasone, a synthetic glucocorticoid, with abnormal suppression indicating potential Cushing syndrome. Similarly, evening salivary cortisol measurement capitalizes on the normal circadian rhythm of cortisol secretion, where elevated late-night salivary cortisol levels can indicate abnormal cortisol production. In contrast, the blood glucose tolerance test is designed to evaluate glucose metabolism and is not used to assess cortisol levels or the effects of cortisol excess. Hence, it does not provide the necessary information to confirm or rule out the presence of Cushing syndrome, making it distinct from the confirmatory tests mentioned.