Q & A: How is the Urinary Cortisol Creatinine Ratio Calculated?

This patient is a 5.5-year-old male neutered Westie that presents with the main complaints of polyuria and polydipsia (PU/PD). The results of a CBC and serum chemistry panal were normal. Strangely, however, the dog has glucosuria with no hyperglycemia. We have run multiple samples on different days of both blood and urine, all showing positive glucosuria but normoglycemia.

I submitted a urine cortisol:creatinine ratio (UCCR)to screen for Cushing's syndrome. It was not the ideal, home-caught, first in AM urine sample. It was free catch urine sample collected here at the clinic. Results show a UCCR ratio of 115. But the urine cortisol is 8.5 μg/dl (normal for Idexx laboratories reference range) and the creatinine is 23.1 mg/dl (low for the reference range). So, am I correct to interpret that the ratio is high because the creatinine is low and not because the cortisol is high? Can Cushing's syndrome be ruled out, or should I proceed to further Cushing's diagnostics?

My Response:

We do the UCC-ratio to correct for increased/decreased filtration and we therefore ought to look at the ratio, not the individual results. Therefore we cannot rule out Cushing's syndrome based on this result. You could, however, repeat measurements on a few home collected, morning samples. It is interesting that the serum alkaline phosphatase is normal in this, which, although still possible, makes hyperdrenocorticism less likely. It still is an important differential though. You could also run a fructosamine to check for chronic relative hyperglycemia.