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Free cortisol in urine represents a direct filtration fraction of blood free cortisol and tends to parallel the cortisol production rate. Over 98% of patients with Cushing's syndrome have elevated baseline urine free cortisol. Because this test has a higher sensitivity and specificity than the overnight dexamethasone suppression test, urine free cortisol is the test of choice when screening for Cushing's syndrome. Patients with Cushing's syndrome have urine free cortisol levels of 1.5 to more than 30 times the upper limit of the reference range. Obese patients usually have normal urinary free cortisol. Because chronic alcohol abuse also causes hypercorticolism, alcoholic patients should refrain from drinking at least 1 month before testing. If patients with depression, significant stress, and alcoholism are excluded, an elevated urine free cortisol has a sensitivity and specificity of close to 100% for Cushing's syndrome. The main drawback of this test is the timed 24-hour urine collection. Because urine free cortisol has no validity in patients with renal failure and a glomerular filtration rate < 30 mL/min, renal function should be checked before testing. Although urine free cortisol may fall below the reference range, it is of little clinical use in evaluating adrenal reserve. However, urine free cortisol is an accurate way to access the adequacy of corticosteroid therapy in patients with chronic adrenal cortical insufficiency. Twenty four hour levels should be within the reference range. The adult (18 years or older) reference range is 24 108 ug/24 hours. The upper limit of normal for children between 0 and 17 years old is 91 ug/24 hours. Specimen requirement is a 24 hour urine collected in a container with 25 mL of 50% acetic acid. Fifteen mL of acid should be added to containers for children < 5 years old. |
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