EquineReview: September–October 2022

Basal adrenocorticotropic hormone (ACTH) is a widely used by practitioners in an attempt to diagnose pituitary pars intermedia dysfunction (PPID; equine Cushing's disease). To determine diagnostic test accuracy, Meyer et al (2021) undertook a systematic review and meta-analysis of the published literature. They then used disease prevalence data to evaluate its diagnostic value in horses with and without clinical signs of PPID.

The group identified 11 studies, but identified numerous areas of potential bias and lack of precision. While the overall sensitivity (true positive rate; 72%) and specificity (true negative rate; 88%) make the test look valuable in clinical practice, the population level data should give us a reason to rethink what cases we test. These showed that in horses without clinical signs of PPID, you would expect almost 10 times more false positives than true positives in horses under 10 years of age (127 and 13 horses per 1000 tested respectively), while in those older than 15, but without clinical signs, the false positive and true positive rate would be similar (104 and 132 horses per 1000 horses respectively). In horses with clinical signs of PPID, the false positive rate is negligible. However, the false negative rate is a cause for concern (306 horses per 1000).