Deficiencies in immunoassay methods used to monitor serum Estradiol levels during aromatase inhibitor treatment in postmenopausal breast cancer patients

Optimal care for breast cancer patients undergoing aromatase inhibitor (AI) treatment is ensured when estradiol (E2) levels are adequately suppressed. To assess treatment efficacy accurately, it is important to measure the serum E2 levels using a well validated assay method with high sensitivity and specificity. This translates into the urgent need to evaluate various E2 immunoassay kits, which are frequently used in hospital settings to measure E2 serum levels in patients undergoing AI treatment, so clinicians obtain accurate and reliable measurements allowing appropriate clinical decision making. Our objective was to evaluate the performance of different commercially available and commonly used E2 immunoassay kits regarding measurement of E2 levels in the serum of postmenopausal breast cancer patients treated with AIs, in comparison to a highly accurate and reliable mass spectrometry assay. Clinical and demographic data were obtained from 77 postmenopausal breast cancer patients who were treated with an AI. Serum E2 levels were measured by 6 immunoassay methods and by liquid chromatography-tandem mass spectrometry (LC-MS/MS), which served as the standard for comparison. Analysis of E2 by LC-MS/MS showed that 70% of the samples had levels that were <5 pg/ml. Three of the assays carried out with commercial E2 immunoassay kits had poor sensitivities and were not able to detect E2 levels <10 or <20 pg/ml. Although two of the E2 assays using commercial kits demonstrated a better sensitivity (5 pg/ml), the measured E2 values were substantially higher than those obtained by LC-MS/MS. The assay with the sixth commercial E2 kit grossly underestimated the true E2 values. E2 assays carried out with commercial E2 immunoassay kits lack the accuracy to measure the very low serum E2 levels found in patients being treated with AIs. Serum samples from such patients should be sent to laboratories that use a mass spectrometry assay.