Comparison of radioimmunoassay and chemiluminescent assay methods to estimate canine blood cortisol concentrations

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Abstract
Background Non‐radioactive assay methods are widely used in commercial laboratories to measure canine blood cortisol concentrations, despite a paucity of published validity data of these tests compared with the traditional ‘gold standard’ radioimmunoassay. Objectives To compare a commercial chemiluminescence assay with radioimmunoassay for blood cortisol measurement, determine the effect of storage on the radioimmunoassay, and determine the impact of any differences on clinical decisions. Methods The study included 54 client owned dogs undergoing adrenal function testing. Fresh plasma or serum samples (n = 170) were assayed for cortisol using radioimmunoassay (RIA1). Samples (n = 196) were also frozen and stored in batches, and assayed by chemiluminescence and radioimmunoassay (RIA2). Results Overall, there was a strong correlation (r2 = 0.967, P < 0.001) between RIA2 and chemiluminescence concentrations without significant difference between means. Strong correlations were present for RIA2 and chemiluminescence at concentration subgroups of > 400 nmol/L (r2 = 0.869, P < 0.001), < 100 nmol/L (r2 = 0.790, P < 0.001), and < 40 nmol/L (r2 = 0.738, P < 0.001). Significant differences between means were present for RIA2 and chemiluminescence concentrations in the < 100 nmol/L, and < 40 nmol/L (P < 0.001) groups. Despite a significant difference in RIA1 and RIA2 results overall, there was no significant difference between RIA1 and RIA2 for any of the concentration groups. In seven cases, discrepant RIA2 and chemiluminescence results may have altered clinical decisions. Conclusions Although RIA and chemiluminescence cortisol concentrations appear highly correlated, a significant difference may exist for concentrations less than 100 nmol/L in stored canine sera. Results of chemiluminescence cortisol assays should be interpreted with caution unless the specific assay method in the laboratory has been adequately validated in dogs.