Pituitary pars intermedia dysfunction ("Equine Cushing's") in horses is most often due to hyperplasia or an adenoma of the pars intermedia of the pituitary gland.
Measurement of endogenous ACTH concentrations using seasonal reference intervals has largely superseded the overnight dexamethasone suppression test as the screening test of choice for PPID. There is seasonal variation in the baseline serum ACTH concentrations in normal horses and ponies, and appropriate seasonal reference intervals must be used in interpretation. The overnight dexamethasone suppression test may still be used outside of the autumn period (a high rate of false positives occurs with this test during the autumn), as both tests perform relatively similarly during the non-autumn period.
Sensitivity is an issue for both tests and retesting in 3-6 months is recommended if results are inconclusive despite a suspicious clinical picture.
An ACTH stimulation test has been described for horses but this is not recommended as a diagnostic test. It does not adequately differentiate normal horses from those with PPID.
Plasma (separated from red cells)
Plain container with no additive
Standard venepuncture. Draw blood into an EDTA tube. Spin immediately or use gravitational separation, then pour off the plasma into a tube or container with NO additives (e.g. yellow-top sterile pot). Red top blood tubes contain a clot activator and must not be used. ACTH degrades rapidly if exposed to heat. Refrigerate the plasma immediately and keep chilled during transportation. If the sample will be delayed, freeze the plasma and transport with ice packs. DO NOT freeze the unseparated EDTA sample. This will make the sample unsuitable for testing.