Question: Why is creatinine measured in a 24 hr urinary cortisol test?

Answer: The reason that urinary creatinine is measured in all patients having a 24 hr urine collection (regardless of what analyte is being measured) is to ensure that the collection is adequate and can be compared to the adequacy of future collections. Urinary creatinine is consistent in patients with stable renal function and is a function of muscle mass. For example, if a 60 kg (132 lb) woman has a urinary creatinine of 2.3 gm, then the physician knows that it was an excessive collection (beyond 24hr) unless of course she has a muscle mass far above normal. In contrast, a collection of .500 gm would be inadequate. Sometimes the clearance rate of an analyte needs to be assessed. Urine cortisol is calculated by some in mcg cortisol/mg creatinine. The evidence that this provides any improvement in the accuracy of the test is scant.

By Dr. James Findling, Endocrinology, Medical College of Wisconsin, Milwaukee, WI, Spring, 2017