Clinical implications of imprecise sampling time for 10- and 30-min thyrotropin-releasing hormone stimulation tests in horses.
Authors: Vorster Dante M, Wang Wenqing, Kemp Kate L, Bamford Nicholas J, Bertin François-René
Journal: Equine veterinary journal
Summary
# Editorial Summary: TRH Sampling Precision in PPID Diagnosis The thyrotropin-releasing hormone (TRH) stimulation test—available in both 10- and 30-minute protocols—remains a cornerstone diagnostic tool for pituitary pars intermedia dysfunction (PPID), yet sampling delays of just one minute can introduce variability in plasma ACTH measurements that may alter clinical interpretation. Vorster and colleagues conducted an in vivo comparison of sampling precision by measuring ACTH concentrations at ±1-minute intervals around both the 10- and 30-minute timepoints in 15 control and 12 PPID-affected horses, using statistical analysis to assess whether timing imprecision changed diagnostic classification. Whilst imprecise sampling produced measurable variability in ACTH concentrations, no statistically significant differences emerged between protocols; however, diagnostic misclassification occurred in 11% of horses overall, with the 30-minute protocol generating false-negative results in one horse and equivocal results in 42% of PPID cases that would only be correctly identified when clinical signs of PPID were considered alongside laboratory findings. For equine practitioners, this emphasises that neither protocol offers sufficient robustness to tolerate casual timing, and that rigid adherence to precise sampling windows—combined with careful evaluation of clinical context such as hirsutism, regional adiposity, and performance changes—remains essential to avoid both false-negative and false-positive diagnoses that could delay treatment or unnecessarily medicalise healthy horses.
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Practical Takeaways
- •Precise timing during TRH stimulation tests remains critical—sampling ±1 minute from protocol time can alter diagnostic classification in approximately 1 in 9 horses
- •Clinical context matters: equivocal ACTH results should be interpreted alongside supportive clinical signs rather than in isolation, as this affects diagnosis reliability
- •Both 10- and 30-min protocols show similar vulnerability to sampling timing errors; practitioners should prioritize accurate timing regardless of which protocol is used
Key Findings
- •Imprecise sampling time (±1 min from protocol target) resulted in variable ACTH concentrations but no significant absolute differences within or between 10- and 30-min TRH protocols
- •Imprecise sampling changed PPID diagnostic classification in 11% (3/27) of horses for both protocols
- •Using 30-min protocol as reference, 42% (5/12) of horses showed equivocal results that would be considered positive when clinical signs were present, compared to 8% (1/12) negative results