2017 ACVIM Forum Research Abstract Program

Based on the results of this study, the TRH stimulation procedure was required in 89% of enrolled horses for laboratory confirmation of PPID. In the sport horse, suspensory desmitis was significantly associated with PPID + status. Veterinarians should include PPID in the list of differential diagnoses when examining sport horses with suspensory desmitis along with early and advanced clinical signs of PPID. To assess measurement of prolactin (PRL) concentration as a screening test for PPID, serum PRL concentration was compared between 31 horses that were enrolled in and 26 horses that failed inclusion criteria for the clinical efficacy study for pergolide. Enrollment required horses to have a hypertrichosis score ≥1 and either an elevated plasma ACTH concentration (≥ 50 pg/mL) or abnormal overnight dexamethasone suppression test results. PRL was measured again after 3 and 6 months of treatment with pergolide in the horses enrolled in the study. PRL concentrations were compared by rank sum analysis and an ANOVA on ranks for repeated measures. A ROC curve was constructed to assess sensitivity and specificity of PRL at various cut-off values as a screening test for PPID. PRL was greater (P = 0.02) in enrolled (7.0 AE 8.2 ng/mL, mean AE SD) as compared to excluded (4.3 AE 2.6 ng/mL) horses. Area under the ROC curve was 0.68 (95% CI: 0.55-0.80). Using cut-off values of ≥3.5 and ≥4.0 ng/mL, sensitivity and specificity were 77 and 54% and 65 and 54%, respectively. Curiously, PRL tended to decrease after 3 months of pergolide treatment (3.8 AE 1.8 ng/mL) but the only significant difference was a higher (P < 0.01) PRL after 180 days of pergolide treatment (9.7 AE 6.4 ng/mL) as compared to the 90 day treatment value. In this cohort of horses, measurement of PRL did not appear to be a useful screening tool for PPID. Further, treatment of PPID-affected horses with pergolide did not decrease PRL. Pituitary pars intermedia dysfunction (PPID) has been considered the most common endocrinologic disorder of aged horses. The diagnosis of PPID is typical based on history, clinical signs and demonstration of endocrinologic dysfunction via diagnostic assay results. The most common diagnostic test utilized by veterinary practitioners for the diagnosis of PPID is currently measurement of basal plasma adrenocorticotropic hormone (ACTH) concentration. Determination of resting plasma ACTH concentration has been the most practical diagnostic test for PPID likely due to the ease of collection and the availability of seasonal reference ranges that allow for testing any time of the year. Multiple recommendations exist concerning the in vitro stability of ACTH. Most all recommendations state samples should be chilled promptly after collection. Multiple recommendations exist for the timing of centrifugation of chilled samples from sample collection. In the US, a discordance exists between laboratory recommendations concerning centrifugation time from sample collection; anywhere from 2 hours up to 4 hours from sample collection. Realistically, many equine ambulatory practitioners do not have the ability to centrifuge the sample within 2 to 4 hours. Based on this dilemma, the purpose of this study was to determine the stability of ACTH levels in blood samples with increasing duration from collection prior to centrifugation.