Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (846 page)

   Assisting in diagnosing acquired or inherited abnormalities of 11β-hydroxy steroid dehydrogenase (cortisol-to-cortisone ratio)

   Diagnosis of pseudohyperaldosteronism due to excessive licorice consumption

   Interpretation

   Cortisol free urine, 24-hour, is increased in Cushing syndrome. The patient can be assumed to have Cushing syndrome if basal urinary cortisol excretion is more than 3 × the upper limit of normal and one other test is abnormal.

   This result is found in 95% of cases of Cushing syndrome. Values <100 μg/24 hours exclude the diagnosis, and values >300 μg/24 hours confirm the diagnosis. If values are intermediate, a dexamethasone suppression test is indicated.

   It is decreased in adrenal insufficiency.

   Limitations

   Urinary cortisol may be detected by antibody-based (immunoassays) or structurally based (HPLC-MS) tests, and the immunoassays may be less specific because antibodies may cross-react with similar steroids.

   An increase is the most useful screening test (best expressed as per gram creatine, which should vary by <10% daily; if variation is >10%, two more 24-hour specimens should be collected). Values should be measured in three consecutive 24-hour specimens to ensure proper collection and account for daily variability, even in Cushing syndrome.

   Increased values may occur in depression, chronic alcoholism, eating disorders, and polycystic ovary syndrome but do not exceed 300 μg/24 hours.

   Various drugs (e.g., carbamazepine, phenytoin, phenobarbital, primidone) will falsely elevate free cortisol levels.

   Acute and chronic illnesses can increase free cortisol levels.

   Renal disease due to decreased excretion may falsely lower the levels of free cortisol.