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The use of a urine cortisol / creatinine ratio is typically done on a single urine sample, rather than a timed urine collection. This is done to compensate for the amount of dilution of the urine. On the other hand, when a timed urine collection is done, the usual measurement is of total free cortisol. With a timed urine collection and calculation of the total cortisol, the amount of dilution does not matter. A timed urine collection is usually done over 24 hours to account for the significant diurnal variation in cortisol levels, so a 12 hour collection cannot simply be doubled.
The current guidelines for diagnosis of Cushing syndrome recommends the use of a 24 hour urine free cortisol, rather than a cortisol/creatinine ratio.
However, when a single urine sample is used to determine the cortisol / creatinine ratio, the concentration of each result is simply divided. So the ratio = concentration of cortisol / concentration of creatinine. See http://www.ncbi.nlm.nih.gov/pubmed/23565435.
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