How do 24-hour urine creatinine levels affect your interpretation of 24-hour urinary calcium or cortisol studies?

24-hour urine is not so easy for patients to collect. They may end up collecting urine for less than 24 hours or may include the specimen obtained after the 24 hours are up, thus collecting an inadequate specimen or collecting too much urine. Creatinine is tested to determine the adequacy of the sample. A better approach is to calculate the Cortisol/Creatinine ratio or the Calcium Creatinine ratio. The results can be calculated and expressed as mg/G creatinine. The calculated ratio can then be compared to the reference range provided by the lab. In essence, Standardizing to Creatinine provides a more reliable and comparable measure of Cortisol or Calcium excretion than simply looking at the total amount of the 24-hour urine sample.

The amount of creatinine will give an indication of how adequate the 24-hour collection is. Occasionally, it is less than 24 hours.