As a screening test for Cushing syndrome, the physician wishes to see whether a patient exhibits normal diurnal rhythm in his or her cortisol secretion. At what time should the specimens be drawn for plasma cortisol determination?

As a screening test for Cushing syndrome, the physician wishes to see whether a patient exhibits normal diurnal rhythm in his or her cortisol secretion. At what time should the specimens be drawn for plasma cortisol determination?

• A. 6 A.M., 2 P.M.
• B. 8 A.M., 4 P.M.
• C. 12 Noon, 6 P.M.
• D. 12 Noon, 12 Midnight

Answer: (B)

Cortisol has a clear daily pattern: it peaks in the early morning after waking and falls through the day toward the evening. To screen for a normal diurnal rhythm, you want one sample from the time of the peak and another from a later time to show the expected decline. Sampling at 8 AM captures the morning peak, and a second sample at 4 PM shows the afternoon drop, making it easy to see whether the rhythm is preserved. Other time pairs either miss the best representation of the peak or don’t clearly capture the decline, so they’re less informative for assessing diurnal rhythm.

Cortisol has a clear daily pattern: it peaks in the early morning after waking and falls through the day toward the evening. To screen for a normal diurnal rhythm, you want one sample from the time of the peak and another from a later time to show the expected decline. Sampling at 8 AM captures the morning peak, and a second sample at 4 PM shows the afternoon drop, making it easy to see whether the rhythm is preserved.

Other time pairs either miss the best representation of the peak or don’t clearly capture the decline, so they’re less informative for assessing diurnal rhythm.