Fluorescent Immunoassay-Established Reference Intervals and Circannual Variation of Equine Plasma Adrenocorticotropic Hormone Concentrations in the Middle East

Background: Adrenocorticotropic hormone is commonly measured to diagnose Pituitary Pars Intermedia Dysfunction (PPID) in horses. Diagnosis requires a validated analyzer and understanding of physiologic variations that affect hormone concentrations. Two commercially available ACTH analyzers are used in equine laboratories. Both are validated for detection of ACTH in equine plasma, but reference intervals have been established for the chemiluminescent analyzer only.

Objectives: Determine reference intervals for ACTH in adult horses via fluorescent immunoassay. Determine the effect of photoperiod on ACTH secretion in the Middle East. Establish normal reference intervals for all plasma ACTH in all months of the year in the Middle East.

Materials and methods: 60 healthy adult horses sampled once; 15 healthy adult horses sampled monthly for 12 months. A single timepoint evaluation of plasma ACTH, then a longitudinal observational repeated study of 15 horses sampled for 12 consecutive months.

Results: Reference intervals-10.3 pg/ml to 32.6 pg/ml. Monthly means and standard deviations lay within the established intervals; however significant differences between individual monthly means were identified.

Conclusion: The AIA-360 analyzer may be used to measure ACTH in horses and this study presents a large-sample reference range that may be used as a guideline in other laboratories. The attenuated photoperiod variability observed in the Middle East is associated with attenuation of the circannual rhythm of ACTH secretion. Significant seasonal variation in mean ACTH concentration was identified but the magnitude of fluctuation was small compared to previous reports. All monthly reference intervals lay within the initial interval, so seasonal adjustment is not needed when sampling horses in peri-equatorial regions.