The overnight dexamethasone has traditionally been used as a screening test for suspected Pituitary Pars Intermedia Dysfunction (PPID, Equine Cushing's). It has largely been superseded by measurement of endogenous plasma ACTH, due to the relative ease of testing (only one visit required for ACTH testing), the development of seasonal reference ranges, and the risks of laminitis associated with dexamethasone injection in at-risk horses. ACTH testing can be performed at any time of the year although test performance is highest in the autumn. Both non-PPID and PPID horses display an increase in ACTH concentrations in the autumn (March, April, May) and therefore seasonal reference intervals appropriate for the time of year must be used.
There is also seasonal variation in results of the dexamethasone suppression test (both in baseline and end results) in normal horses and ponies. There are also differences in the baseline and end stage serum cortisol concentrations between horses and ponies. Test results must always be interpreted with clinical signs.
Plain (red top) or Gel (serum separator) tube
- Collect a baseline serum sample (red top tube) between 4-6pm
- Administer dexamethasone at 40ug/kg IM (0.04mg/kg)
- Collect a second serum sample at noon the next day (about 19 hours post-dexamethasone)
- Submit both samples to the laboratory for cortisol analysis