Cortisol is a stress hormone that is excreted from the body in the urine. The amount of cortisol in the urine reflects the average cortisol concentration in the blood at the time that the urine was formed. However, this measurement is affected by the concentration of the urine. Creatinine is a product of muscle metabolism and it is normally lost in the urine at a relatively steady rate. Because of this, the ratio of cortisol to creatinine in the urine can be used to account for the effect of urine concentration.
Why is this test done?
The urine cortisol/creatinine ratio is usually evaluated in animals suspected of having Cushing’s disease (hyperadrenocorticism). When an animal has hyperadrenocorticism (Cushing’s disease or syndrome) it produces an excessive amount of cortisol. This increased blood cortisol concentration results in increased loss of cortisol into the urine.
The urinary cortisol/creatinine ratio is therefore usually increased in animals with Cushing’s disease
Does an increased urine cortisol/creatinine ratio always mean that Cushing’s disease is present?
No, there are other causes of an increased urinary cortisol/creatinine ratio.
Simple stress, such as a car ride or a visit to your veterinarian, may cause a mild increase in this ratio. The presence of other illnesses may also result in increased cortisol production by the adrenal glands and thereby increase this ratio.
However, if your pet has appropriate clinical signs and other initial screening tests (CBC, urinalysis, and biochemical profile) are also supportive of Cushing’s disease, , then further confirmatory testing for Cushing’s disease is indicated. These further tests may include the ACTH stimulation test and/or the dexamethasone suppression test.
What does a normal urine cortisol/creatinine ratio mean?
Like any other test, the urine cortisol/creatinine ration result must always be evaluated along with other clinical information and data. However, if the cortisol/creatinine ratio is not elevated, then it is very unlikely that your pet has Cushing’s disease.
Exactly how is the urine cortisol/creatinine ratio test done?
This test involves the collection of a single urine sample, taken first thing in the morning.
Ideally, this sample should be collected at your home, to minimize the effects of stress due to a hospital visit. By collecting a first morning sample, the average amount of cortisol that has been lost into the urine overnight can be assessed. The urine sample is usually sent to a referral lab for analysis.