In many cases of pituitary pars intermedia dysfunction (PPID or Cushing’s Disease), overt clinical signs point to dysfunction within the pituitary gland. In less demonstrative cases, blood testing is necessary to confirm a diagnosis.
A screening test evaluating baseline ACTH concentration is often used as a preliminary check on the hormonal status of a horse.
A study sought to identify the accuracy of using ACTH as a biomarker for PPID [Meyer, J.C.; Hunyadi, L.M.; Ordonez, J.M. The accuracy of ACTH as a biomarker for pituitary pars intermedia dysfunction in horses: A systematic review and meta-analysis. Equine Veterinary Journal Aug 2021; DOI: 10.1111/evj.13500].
Obtaining an accurate diagnosis is important to facilitate treatment of a horse with PPID with pergolide or to eliminate unnecessary treatment. An ACTH level is suggested by the Equine Endocrinology group as a “rule-in” test for horses with clinical signs of PPID, particularly if those signs are moderate to advanced. In the absence of a positive ACTH test, then those horses are subjected to a TRH stimulation test.
This study extracted data from 752 adult horses or ponies to identify true-positives, false-positives, true-negatives and false-negatives. Those with PPID numbered 234 while 518 horses did not have PPID.
In summary, the authors conclude, “The evidence indicated that ACTH as a diagnostic test for PPID in the horse is not sufficient unless accompanied by classical clinical signs. ACTH should not be used as a screening test or as the sole diagnostic test in horses not exhibiting clinical signs of PPID.”