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

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.

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 °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.

There was a significant effect of time on ACTH concentrations in horses with PPID at 4 and 20 °C and in all horses at 20 °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 °C for 24 or 48 hours.

Time but not storage of plasma in serum tubes decreases the measured ACTH concentration in horses with PPID.

Transferring plasma into serum tubes does not significantly alter the measured ACTH concentrations; however, time between collection and analysis might lead to erroneous diagnoses.