Dexamethasone suppression test - overnight

Dexamethasone suppression test – overnight

Species: Equine

Specimen: Serum

Container: Red top or gel (serum separator) tube

Collection protocol:

1.    Collect a baseline serum sample (red top tube) between 4-6pm

2.    Administer dexamethasone at 40ug/kg IM (0.04mg/kg)

3.    Collect a second serum sample at noon the next day (about 19 hours post-dexamethasone)

4.    Submit both samples to the laboratory for cortisol analysis

Special handling/shipping requirements: Send all samples to the laboratory within 12-24 hours of collection.

General information about the disease:

Pituitary pars intermedia dysfunction (PPID) in horses is most often due to hyperplasia or an adenoma of the pars intermedia of the pituitary gland. The overnight dexamethasone suppression test was previously the test of choice for diagnosis but has largely been superseded by measurement of endogenous ACTH. Occasionally the suppression test is used when the endogenous ACTH is equivocal. An ACTH stimulation test has been described for horses but this is not recommended as a diagnostic test as it does not adequately differentiate normal horses from those with hyperadrenocorticism.

There is seasonal variation in the baseline serum ACTH concentrations and also in the 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 therefore should be interpreted with clinical signs.

General information about when this test is indicated:

Diagnostic work up for suspected PPID

Comparison with other related tests:

Endogenous ACTH has largely superseded the overnight dexamethasone suppression test as the screening test of choice for the diagnosis of PPID. 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) compared with other times of the year but the increase is more exaggerated in PPID cases. Seasonal reference intervals appropriate for the time of year must be used. The same tests can be used in donkeys.