Background : Serum E2 must be monitored for aromatase inhibitor (AI) therapy, but conventional assays lack sensitivity. Subjects/Methods : Forty amenorrheic breast cancer patients scheduled for AI treatment but requiring hormonological confirmation of their menopausal status were studied. Serum E2 data generated by high-sensitivity RIA and by LC-MS/MS were analyzed for correlation. Results : RIA gave a higher E2 value than LC-MS/MS in 62% of cases, but there was a significant positive correlation. Patients whose E2 levels by RIA were≥2.5 pg/mL higher than those by LC-MS/MS (RIA-H group) and all other patients (RIA-N group) were compared. Both groups showed strong correlations between the two assay methods. With both methods patients with a high BMI had significantly elevated E2. Multiple regression analysis used age, age at menarche, number of births and BMI as explanatory variables. Significant variables were the BMI with LC-MS/MS, and both BMI and age with RIA. The RIA-H and RIA-N groups showed no difference in regard to the BMI, whereas the age was significantly lower in the RIA-H group. Summary : Serum E2 levels determined for postmenopausal women by RIA and LC-MS/MS generally correlated well. High-sensitivity RIA is a potentially useful clinical assay, but it overestimated serum E2 in some women
