Background
Estradiol (E2) is a commonly requested test. The ability to measure very low concentrations is relevant when assessing gonadal function in children, men, postmenopausal women on hormone replacement therapy and breast cancer patients treated with aromatase-inhibitors. Accuracy, specificity, sensitivity and reproducibility are critical. Various methods for measuring E2 exist, however, the gold standard is isotope-dilution liquid-chromatography-tandem mass spectrometry (LC-MS/MS). 1,2
Methods
At Pathology Queensland E2 is measured via a chemiluminescent immunoassay on the Diasorin-Liaison instrument. We performed a comparison of 356 samples submitted for routine E2 analysis with both LC-MS/MS and immunoassay to determine whether they meet the analytical quality specifications at low concentrations. A total allowable error of 27% based on biological variation is the minimum acceptable quality specification, allowing differentiation between 100 and 50pmol/L.
Results
356 samples were analysed. Of these, 274 had a Liaison result above the Liaision’s lower reporting limit (36pmol/L). Despite the acceptable correlation and regression slope of 1 for the entire group a significant constant bias of 30pmol/L was detected. Numerous Liaison results were outside the allowable difference limits, particularly at low concentrations.
Results between the Liasion’s lower reportable limit and 300pmol/L (n=215) were examined separately and the allowable error tolerance was further relaxed to ±25pmol/L below 100pmol/L. Liaison demonstrated a positive bias of 31pmol/L and many results remained outside the extended allowable tolerance limits. We mathematically corrected the constant bias of the Liasion E2 results and repeated the statistical analysis of differences. The number of discordant results below 150pmol/L remained significantly elevated (p <0.001). After correcting for bias a result of 100pmol/L will have a 95% confidence distribution of ±60pmol/L, which is more than double the minimum acceptable ±27pmol/L.
Conclusion
Although automated immunoassay methodology has improved with lower detection levels, E2 can only be reliably measured by LC-MS/MS at very low concentrations.