Plasma adrenocorticotropin concentrations vary with assay used in horses with and without pituitary pars intermedia dysfunction.

Overview

• This study compared two different laboratory assays used to measure plasma adrenocorticotropin (ACTH) levels in horses, both healthy and those with pituitary pars intermedia dysfunction (PPID), to see how the choice of assay influences diagnosis results.

Background

• Adrenocorticotropin (ACTH) is a hormone measured in blood plasma to help diagnose PPID, a common endocrine disorder in older horses.
• ACTH levels can vary depending on the season and the type of laboratory test (assay) used.
• Pituitary pars intermedia dysfunction (PPID) is diagnosed by elevated plasma ACTH and clinical signs such as hypertrichosis (excessive hair growth).
• Thyrotropin releasing hormone (TRH) stimulation test amplifies ACTH secretion to help identify PPID.

Purpose of the Study

• To compare two assays for measuring plasma ACTH concentrations: the Scantibodies immunoradiometric assay (IRMA) and the Immulite 2000Xpi chemiluminescent immunometric assay (CIA).
• To determine how these assays perform in different groups of horses: young healthy, aged healthy, and horses with diagnosed PPID.
• To evaluate the impact of assay choice on baseline ACTH and TRH-stimulated ACTH levels and thus on PPID classification.

Methodology

• Three groups of horses were studied:
  • Four young horses (average age 6 years) without hypertrichosis and low pituitary pathology scores (1-3 out of 5).
  • Eight aged horses (average age 26 years) without hypertrichosis and moderate pituitary pathology scores (2-3 out of 5).
  • Five horses diagnosed with PPID (average age 27 years), all showing hypertrichosis and severe pituitary pathology (score 5 out of 5).
• ACTH levels were measured at baseline and 10 minutes after intravenous administration of TRH during non-fall months to reduce seasonal variability.
• Horses were euthanized within seven days of testing for post-mortem pituitary gland histopathological confirmation.
• Plasma ACTH was measured with both assays (IRMA and CIA) on each sample to compare results directly.

Key Findings

• Baseline elevated ACTH (>40 pg/mL) detection:
  • IRMA detected elevated ACTH in 0/4 young, 2/8 aged, and 4/5 PPID horses.
  • CIA detected elevated ACTH in 0/4 young, 4/8 aged, and 4/5 PPID horses.
• Post-TRH stimulation elevated ACTH (>200 pg/mL):
  • IRMA detected elevations in 0/4 young, 0/8 aged, and 2/5 PPID horses.
  • CIA detected elevations in 1/4 young, 5/8 aged, and 5/5 PPID horses.
• Mean increases in ACTH after TRH stimulation were significantly higher with CIA than IRMA:
  • Approximately 13-fold higher in young horses.
  • Approximately 18-fold higher in aged horses.
  • Approximately 14-fold higher in PPID horses.
• This means CIA is more sensitive or gives higher readings of ACTH after TRH stimulation compared to IRMA.

Conclusions

• There are substantial differences in plasma ACTH concentrations measured by different assays, impacting diagnostic classification for PPID.
• The choice of assay affects both baseline and stimulated ACTH results, potentially leading to differences in whether a horse is classified as PPID positive or negative.
• This finding emphasizes the need for assay-specific reference ranges and diagnostic cutoffs when testing horses for PPID.
• Veterinarians and laboratories must consider assay differences and validate their diagnostic thresholds to avoid misdiagnosis.

Implications for Veterinary Practice

• Clinicians should be aware that ACTH test results can differ depending on the assay used.
• Diagnostic criteria for PPID should be established per assay or assays should be standardized if possible.
• Further research could help to harmonize ACTH assays or develop universally accepted diagnostic thresholds.