Substantial discrepancies in 17beta-oestradiol concentrations obtained with three different commercial direct radioimmunoassay kits in rat sera

The extensive use of oestrogen for contraception and amelioration of post-menopausal symptoms has made it the subject of substantial recent research efforts, and ovariectomized (ovx) rats treated with exogenous ovarial hormones are important when investigating the effects and mechanisms of oestrogen actions. The crucial need to control and monitor plasma levels of 17beta-oestradiol calls for accurate, precise and robust assay methods. The performance of direct radioimmunoassays (RIAs) in measurement of 17beta-oestradiol has been reported previously for human samples, but to our knowledge not for rat samples. In the current study, 552 serum samples from ovx, native and hormone-treated rats were used to compare the performance of three commercially manufactured direct RIAs from the companies DPC (Siemens Healthcare Diagnostics Inc., formerly Diagnostic Products Corporation), DSL (Diagnostic Systems Labs) and MPB (MP Biomedicals, formerly ICN Biomedicals). Substantial differences in results between the three assay methods were found when measuring serum 17beta-oestradiol concentrations. The following formulas describing the relation between the different methods were obtained using weighted Deming's orthogonal regression (based on pg/mL): DSL = 0.43*DPC+12.3, MPB = 2.1*DPC+84.7 and DSL = 4.8*MPB+22.2. Furthermore, a preceding diethyl ether extraction step of the serum appears to impair the performance of the RIAs in the present samples (based on pg/mL): DPC(ex) = 0.39*DPC(unex)+0.76, DSL(ex) = 0.32*DSL(unex)-1.7 and MPB(ex) = 0.22*MPB(unex)+1.4.