Instruct the patient to void at 8 AM and discard the specimen. Then collect all urine including the final specimen voided at the end of the 24-hour collection period (ie, 8 AM the next morning) in a plastic bottle. Screw the lid on securely. (It is acceptable to collect cortisol with boric acid or HCl preservative.)
–
USE:
The diagnosis of Cushing syndrome (CS) requires evidence of cortisol hypersecretion. While serum cortisol levels fluctuate unpredictably and are strongly dependent on concurrent cortisol-binding globulin (CBG) levels, a 24-hour urine specimen integrates the cortisol production for an entire day and is not affected by CBG. Urinary cortisol reflects the portion of serum-free cortisol filtered by the kidney, and correlates well with cortisol secretion rate.
The most common causes of CS are pituitary adenoma (65% to 70%), adrenal tumor (15% to 20%), and ectopic CS (10% to 15%).
–
Limitations:
Not useful in the evaluation of adrenal insufficiency (AI). Results in AI overlap with normal ranges. Increased in pregnancy and with oral contraceptives. Increased excretion may be found with pseudo-Cushing syndrome, trauma, or infection.2 Tetracyclines may cause false elevation of results.
This test was developed and its performance characteristics determined by LabCorp. It has not been cleared or approved by the Food and Drug Administration.
