Clinical implications of imprecise sampling time for 10- and 30-min thyrotropin-releasing hormone stimulation tests in horses

Abstract

Background: The thyrotropin-releasing hormone (TRH) stimulation test is used to diagnose pituitary pars intermedia dysfunction (PPID) using 10- or 30-min protocols. Imprecise sampling time for the 10-min protocol can lead to misdiagnoses. Objectives: To determine the effect of imprecise sampling time for the 30-min protocol of the TRH stimulation test. Study design: In vivo experiment. Methods: Plasma immunoreactive adrenocorticotropin (ACTH) concentrations were measured 9, 10, 11, 29, 30 and 31 min after intravenous administration of 1 mg of TRH in 15 control and 12 PPID horses. Differences in ACTH concentrations between sampling times, variability in ACTH concentrations between protocols, and diagnostic classification of PPID were assessed using Friedman's test, Bland–Altman plots, and Fisher's exact test, respectively, with 95% confidence intervals reported and significance set at p < 0.05. Results: Imprecise sampling time resulted in variable ACTH concentrations, but significant differences in absolute ACTH concentrations were not detected for imprecise sampling within each protocol or between protocols. Imprecise sampling changed PPID diagnostic classification for 3/27 (11 [4–28] %) horses for both protocols. Using the 30-min protocol as a reference, 1/12 (8 [1–35] %) horses returned a negative test result and 5/12 (42 [19–68] %) horses returned equivocal test results that would be considered positive in practice due to the presence of supportive clinical signs. Main limitations: Limited sample size and inter-horse variability reduced the ability to detect small but potentially relevant differences. Conclusions: Overall, the impact of imprecise sampling was not significantly different between the 10- and 30-min TRH stimulation test protocols. However, diagnostic classification for PPID would have varied between the 10- and 30-min protocols in this population, if clinical signs had been ignored. Precise timing during TRH stimulation tests and contextual interpretation of ACTH concentrations remain fundamental for the diagnosis of PPID.