Diagnostic accuracy of post-ACTH challenge salivary cortisol concentrations for identifying horses with equine glandular gastric disease1

Abstract The aims of this study were to better characterize the adrenal response to i.v. adrenocorticotropic hormone (ACTH) in horses with and without gastric disease and to validate and simplify the ACTH stimulation test by determining the diagnostic accuracy of six consecutive sampling time points after ACTH administration for equine glandular gastric disease (EGGD) and equine squamous gastric disease (ESGD). Twenty-six endurance and eventing horses without clinical disease [Sport Horse Population (SHP)] and an independent population of 62 horses [General Population (GP)] were grouped by gastroscopic findings (no/mild vs. moderate/severe EGGD, grade 0–1 vs. 2–4 ESGD, respectively) and underwent an ACTH stimulation test. Salivary cortisol (ng/mL) was analyzed before and 30, 60, 90, 120, and 150 min after i.v. injection of 1 µg/kg BW synthetic ACTH1-24. The association between having moderate or severe EGGD or ESGD and the amount of salivary cortisol was analyzed by means of receiver-operating characteristic (ROC) analysis. The following explanatory variables were considered: cortisol values for every time point, the area under the curve (AUC)—including all time points and corrected for the baseline—and the partial areas under the curve AUC0-90 and AUC90-150. Sampling after 60 min had highest association with moderate/severe EGGD. The diagnostic potential of the ACTH test was higher for the SHP [sensitivity 100% (95% CI 54% to 100%), specificity 75% (95% CI 51% to 91%), ROC-AUC 91% (95% CI 69% to 98%), 1-sided P-value < 0.001] than for the GP [sensitivity 75% (95% CI 48% to 93%), specificity 52% (95% CI 37% to 67%), ROC-AUC 68% (95% CI 51% to 79%), 1-sided P-value = 0.0064]. There were, however, no significant associations with ESGD. The superiority of sampling after 60 min suggests that the initial release of cortisol rather than its peak or the AUC are relevant regarding EGGD. Even though the wide confidence intervals and thus the lack of diagnostic accuracy do not presently support clinical use, characterization of the adrenal response to an ACTH stimulus improves the understanding of EGGD pathophysiology and its relation to stress.