Time but not storage of equine plasma in silicate-coated tubes affects adrenocorticotropic hormone stability.

Abstract

OBJECTIVE: Pituitary pars intermedia dysfunction (PPID) is diagnosed by measuring the ACTH concentrations. Due to the reported instability of ACTH, it is recommended to transfer centrifuged plasma into cryovials; however, in practice, cryovials are infrequently used, and serum (red-top) tubes are used instead. This study investigated whether this procedure affects ACTH concentrations and the diagnosis of PPID. METHODS: This was a cohort study. Blood was collected into EDTA tubes from 9 horses with PPID and 7 controls. After centrifugation, plasma was either aliquoted into a cryovial or into a serum tube. Samples were stored at 4 and 20 &#xb0;C with ACTH concentrations measured using a chemiluminescent assay at 2, 24, and 48 hours after collection. Data were analyzed using a 2-way repeated measures ANOVA with P < .05 considered significant. RESULTS: There was a significant effect of time on ACTH concentrations in horses with PPID at 4 and 20 &#xb0;C and in all horses at 20 &#xb0;C with decreasing ACTH concentrations over time; however, no significant effect of storing plasma in a serum tube was detected. Three horses diagnosed with PPID would have had a different classification if the samples had been kept at 20 &#xb0;C for 24 or 48 hours. CONCLUSIONS: Time but not storage of plasma in serum tubes decreases the measured ACTH concentration in horses with PPID. CLINICAL RELEVANCE: Transferring plasma into serum tubes does not significantly alter the measured ACTH concentrations; however, time between collection and analysis might lead to erroneous diagnoses.