Evaluation of two chemiluminescent assays compared with radioimmunoassay for serum progesterone measurement in bitches

Serum progesterone (sP4) measurement is commonly used to determine the optimal time for mating in bitches, and to diagnose reproduction-related abnormalities. Radioimmunoassay (RIA) is the gold standard assay, but is becoming less available, and has several practical disadvantages. Chemiluminescence immunoassays (CLIA) are commonly used in human medicine for sP4 measurement, and are becoming more available in veterinary medicine. Our objective was to compare the sP4 results obtained by RIA and two CLIA systems (Immulite-Siemens [IS-CLIA] and Elecsys-Roche [ER-CLIA]) in the same sera in 60 client-owned healthy bitches at different estrous cycle stages. The agreement between the two CLIAs and RIA was examined using the Passing-Bablok regression and Bland Altman plots. Comparing sP4 concentrations measured by the IS-CLIA to the RIA yielded an intercept of 0.16 ng/mL (95% confidence interval [95%CI], 0.03–0.25) with a slope of 0.45 (95%CI, 0.44–0.47) and a median difference of −2.10 ng/mL (P < 0.0001) that was strongly correlated to the average of measurements (r = −0.97; P < 0.0001). Comparing sP4 concentrations measured by the ER-CLIA to the RIA yielded an intercept of −0.23 ng/mL (95%CI, −0.56 to −0.09) with a slope of 1.06 (95%CI, 1.00–1.12) and a median difference of −0.09 ng/mL (P = 0.9), that was weakly correlated to the average of measurements (r = 0.34; P = 0.018). The performance of the ER-CLIA was similar to the RIA, while the IS-CLIA showed significantly different results compared to the RIA. Our study supports the conclusion that sP4 results generated by the ER-CLIA can be used interchangeably with RIA results for clinical purposes, while IS-CLIA results require adjustment to RIA results for clinical practice.