Agreement of different methods for assessment of progesterone concentrations in beef cattle serum

Progesterone (P4) is responsible for the main reproduction processes. Concentration of P4 varies widely among different determination methods, and interpretation of these values may be difficult. The objective of the current study was to assess the agreement of three different enzyme immunoassays (ELISA) in relation to radioimmunoassay (RIA) of P4 concentration assessment of beef cow serum samples. Samples were collected randomly considering high (pregnant cows) and low (non-pregnant cows) P4 concentrations. Depending on the P4 assessment method, four groups were created as follows: Group 1 - direct samples assessed by ELISA, Group 2 - extracted samples assessed by ELISA, Group 3 - samples assessed by automated ELISA, and Group 4 - samples assessed by RIA. The mean progesterone concentration was 4.50 ng/mL, 1.24 ng/mL, 4.07 ng/mL and 4.39 ng/mL from Group 1 to Group 4, respectively. The mean difference (MD) between Group 1, Group 2 and Group 3 individually compared with Group 4 was -0.10 ± 1.24 ng/mL, 3.15 ± 3.58 ng/mL and 0.33 ± 1.42 ng/mL, and the 95% confidence interval (CI) for the differences (s) was from -0.99 to 0.78 ng/mL, from 0.59 to 5.71 ng/mL, and from -0.69 to 1.34 ng/mL, respectively. The confidence interval for the lower and upper limit of the agreement ranged from -4.12 to -1.05 ng/mL and from 0.84 to 3.91 ng/mL between Group 1 and Group 4, from -8.45 to 0.42 ng/ mL and from 5.88 to 14.75 ng/mL between Group 2 and Group 4, from -4.29 to -0.76 ng/mL, and from 1.41 to 4.94 ng/mL between Group 3 and Group 4. Our findings show that the best agreement with RIA was observed for Group 1 and Group 3, while the agreement in the extraction method was least accurate.