Dexamethasone has variable suppression effects and duration of action in cats compared to dogs. High dose suppression testing is therefore more reliable than low-dose testing, and is the preferred method of screening for Cushing's in cats. Results must always be interpreted in conjunction with clinical signs. Adrenal function testing should not be preformed on a cat with significant systemic illness or that is significantly stressed, which may lead to adrenal resistance and false-positives.
Serum (minimum volume 1-2 ml)
Plain (red top) or Gel tube
- Collect a resting serum sample (red top tube) at zero hours and label it “0 hour”
- Allow tube to clot. If possible separate the serum from the red cells by centrifugation. If a plain tube has been used for the assay, centrifuge and transfer the serum from the collection tube into a plain (red top tube). Store at 4 degrees.
- Inject dexamethasone sodium phosphate intravenously at a dose rate of 0.1 mg/kg of body weight.
- Collect a serum samples at 4 hours post injection, label as “3hr”. Follow separation instructions as above.
- Collect a serum samples at 8 hours post injection, label as “8hr”. Follow separation instructions as above.
Suppression is defined as a cortisol at either 4 or 8 hours that is < 50% of the baseline cortisol or a cortisol concentration at 4 or 8 hours that is <40 nmol/l. Cats with adrenal tumours do not suppress. Some cats with pituitary tumours may not suppress.
At home protocol:
This uses the urinary cortisol to creatinine ratio. This test is considered easier to perform and to interpret than the in clinic test. Diagnostic results for discriminating between pituitary adenoma and adrenal tumours also appear better than results from the in clinic method, although the low dose dexamethasone suppression test is better as a screening test than the cortisol to creatinine ratio.
- The owner collects two consecutive morning urine samples.
- After collection of the second urine sample the owner then administers three oral doses of 0.1 mg/kg/dose of dexamethasone at 8 hourly intervals i.e. 8.00 am, 4 pm and midnight.
- On the third morning a final urine sample is obtained.
- All three urine samples are brought to the clinic and shipped to the laboratory for urine cortisol/creatinine ratio testing. The first two morning samples are used as a screening sample for Cushing’s and provide a basal sample. The third sample is used as a discriminatory test between pituitary adenoma and adrenal tumours. Suppression is described as a ratio that is < 50% of the baseline. Cats that suppress have pituitary adenoma; cats that do not suppress may have either pituitary adenoma or an adrenal tumour.