Abstract: Background: Measuring endogenous plasma adrenocorticotropic hormone (ACTH) is the most common diagnostic test for equine pituitary pars intermedia dysfunction. Accurate diagnosis depends on understanding normal values and ACTH secretion patterns.
Published reference intervals for ACTH are based on costly chemiluminescent assays. An affordable fluorescent immunoassay has recently been adapted for the equine sector. Studies have shown acceptable intra-and-inter-run variation with poor value agreement between analysis methodology.
Additionally, circannual variation with a physiological increase in plasma ACTH as the daily photoperiod shortens has been demonstrated in both the northern and southern hemispheres. This has not been evaluated in the Gulf region, where the circannual photoperiod variability is attenuated compared to other locations
Objectives: To establish fluorescent-immunoassay ACTH reference ranges and evaluate circannual variability
Methods: Potassium-EDTA anticoagulated plasma was analyzed via AIA-360 analyzer, and reference intervals were established by determining normality, then the 2.5th – 97.5th percentile of the dataset. Seasonal variation was assessed with Benjamini-Hochberg corrected pairwise t-tests, to compare the means of monthly datasets.
All monthly means and standard deviations lay within the established reference intervals, however significant differences between individual monthly means were identified.
Conclusions: This is the first reported reference interval for AIA-360 analyzer-generated plasma ACTH in horses. The diminished circannual photoperiod variability in the Gulf appears to attenuate the seasonal variability of ACTH secretion, eliminating the need for seasonally adjusted reference intervals.