Effect of early or late blood sampling on thyrotropin releasing hormone stimulation test results in horses.
Authors: Thane Kristen, Uricchio Cassandra, Frank Nicholas
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: TRH Stimulation Test Timing and PPID Diagnosis in Horses When diagnosing pituitary pars intermedia dysfunction (PPID) using the TRH stimulation test, veterinarians collect blood precisely 10 minutes post-injection to measure ACTH concentration, but the clinical significance of minor timing deviations remained unclear. Kristen and colleagues examined this question by conducting a single TRH stimulation test on 24 healthy adult horses with unknown PPID status, collecting blood samples at 9, 10, and 11 minutes post-injection and analysing ACTH via chemiluminescent immunoassay. Whilst intra-assay variability was minimal at 3% median difference between paired 10-minute samples, early or late collection showed considerably greater variability (median 10%, range 0–92%), with 75% of horses displaying at least one reading differing by ≥10% from the standard 10-minute sample and 21% potentially receiving different diagnostic interpretations. These findings held regardless of underlying PPID status, demonstrating that precise timing during sample collection is essential for reliable diagnostic outcomes. Clinically, this research underscores that even one-minute deviations from protocol can substantively alter test interpretation, making rigorous adherence to sampling protocols non-negotiable when assessing PPID status in suspected cases.
Read the full abstract on PubMed
Practical Takeaways
- •Strict adherence to the 10-minute sampling protocol is essential when performing TRH stimulation tests, as even ±1 minute variation can alter diagnostic interpretation in approximately 1 in 5 cases
- •Clinicians should standardize collection timing and document exact minutes post-injection; small deviations may lead to misdiagnosis of PPID status
- •Consider repeating borderline TRH test results with meticulous timing control before making treatment decisions, especially if clinical signs are equivocal
Key Findings
- •Minor variability (0-6%, median 3%) was observed between duplicate 10-minute samples, establishing baseline intra-assay precision
- •Seventy-five percent of horses (18/24) had at least one early (9-min) or late (11-min) sample differing by ≥10% from the standard 10-minute collection
- •21% of horses (5/24) would receive different clinical interpretation of PPID status based on timing variations of ±1 minute
- •Variability in ACTH response timing was independent of actual PPID disease status (P = 0.59)