Unfortunately, no perfect PPID test (one that is 100% accurate with a single-sample test) yet exists. At the 2006 AAEP Convention, Harold Schott, DVM, PhD, Dipl. ACVIM, professor of large animal clinical sciences at Michigan State University, noted that 11 tests are possible, from simple evaluation of clinical signs (“over-the-fence” diagnosis of hirsutism) to various measures of hormone levels in blood plasma and urine.

“The dexamethasone suppression test (DST) is considered by many to be the gold standard diagnostic test, probably because of experience with it rather than actual data,” he commented. “It’s the most widely accepted test, the samples are stable (less affected by variations in handling), and cortisol measurement is readily available (at labs).”

The test is based on the fact that one pituitary pars intermedia hormone product stimulates the adrenal glands to produce excess cortisol (often termed stress hormone). Schott explained that the DST involves measuring cortisol, giving the horse dexamethasone (a steroid analogue that is used in this case to suppress cortisol stimulation from another lobe of the pituitary gland) in the late afternoon, then measuring plasma cortisol the next morning (15 and 19 hours after dexamethasone administration). Cortisol levels greater than 1 ug/dL at those times support a diagnosis of PPID.

Disadvantages: The DST requires three client visits (although the test can be modified to two visits), it is reported to exacerbate laminitis in rare cases (although Schott noted this observation is poorly documented), its results are not always repeatable, and it might miss early PPID. He briefly discussed several other hormone tests and their accuracy levels, noting that researchers are finding significant seasonal variation in hormone levels and, thus, seasonal variation in test results, even on the same horses.

“The take-home message is that seasonal variation complicates diagnostic testing–endocrine testing is not recommended from mid-August to mid-November because we have difficulty interpreting the results,” he cautioned.

In addition to hormone testing, researchers often will evaluate pituitary gland tissue of research horses post-mortem to try to correlate histological (tissue) characteristics with hormone test results and clinical signs.

Schott described a study that found lesions were common in both the pars intermedia and pars distalis regions of the pituitary gland. There was one other notable feature of the horses that were examined–they were all clinically normal.

“Based on this (‘abnormal’ tissue findings in horses that had no clinical signs of disease), I’m not sure histological examination is the way to go,” he opined. “Another take-home message is that hirsutism is still the most accurate diagnostic feature (identifying 86% of affected horses). So why test horses further? To evaluate their response to treatment!”

Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse’s AAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads