We prepared proficiency testing samples with estradiol concentrations ranging from 20 to 600 ng/L. 1 Specimens were distributed to 430 laboratories enrolled in the endocrinology proficiency testing program; 159 of these submitted results for estradiol testing. The mean CV for all methods was 8.4% at estradiol concentrations >100 ng/L. Greater imprecision (CV = 33–40%; n = 16) at estradiol concentrations ranging from 150 to 400 ng/L was observed for the (AxSYM) MEIA method in the proficiency testing event of September 2001. Results from this method were bimodal in distribution, with apparent centers at 120 and 240 ng/L. 2
We examined the possibility that components of the proficiency testing specimens interfered with the AxSYM method. Steroid components of the endocrinology proficiency testing specimens are listed in Table 1 . We examined their potential to interfere with the AxSYM method by adding estradiol to processed human serum (proficiency testing serum matrix) to a final concentration of 1000 ng/L, together with one of the steroids listed in Table 1 at either of two concentrations (one within and one above the reference intervals). Estradiol in each sample was measured by the AxSYM method in an undiluted sample and a 1:5 automated dilution. (Abbott Laboratories has instructed users of the AxSYM instrument to perform a 1:5 automatic dilution of samples for some specimens.) Of the steroids tested, only unconjugated estriol showed an inhibitory effect on estradiol measurements, in the undiluted specimen. The magnitude of this effect was substantially reduced when the assay was performed with the 1:5 automated dilution protocol.