During a cosyntropin stimulation test, what constitutes an inadequate cortisol response?

• A. No rise or rise less than 18–20 µg/dL
• B. Rise above 40 µg/dL
• C. Increase only after 4 hours
• D. Decrease in cortisol

Answer: (A)

Cosyntropin stimulates the adrenal cortex to release cortisol, so after administration the cortisol level should rise if the adrenals can respond normally. In adults, a peak cortisol level after stimulation of about 18 µg/dL (some labs use 20 µg/dL) is generally considered the threshold for a normal, adequate response. If there is no rise, or the rise is below that threshold, the response is considered inadequate, indicating possible adrenal insufficiency (primary or secondary) with insufficient adrenal reserve.

The option describing no rise or a rise less than 18–20 µg/dL fits this criterion exactly. The other ideas don’t reflect the standard definition: rising to an exceptionally high level (over 40 µg/dL) shows a strong, adequate response; waiting several hours (like 4 hours) isn’t how the test is interpreted or timed; and a decrease in cortisol after stimulation would be inconsistent with the expected physiologic response to cosyntropin.