Ultra-high sensitivity analysis of estrogens for special populations in serum and plasma by liquid chromatography-mass spectrometry: Assay considerations and suggested practices

Estrogen measurements play an important role in the clinical evaluation of many endocrine disorders as well as in research on the role of hormones in human biology and disease. It remains an analytical challenge to quantify estrogens and their metabolites in specimens from special populations including older men, children, postmenopausal women and women receiving aromatase inhibitors. Historically, immunoassays have been used for measuring estrogens and their metabolites in biological samples for risk assessment. However, the lack of specificity and accuracy of immunoassay-based methods has caused significant problems when interpreting data generated from epidemiological studies and across different laboratories. Stable isotope dilution (SID) methodology coupled with liquid chromatography-selected reaction monitoring-mass spectrometry (LC-SRM/MS) is now accepted as the 'gold-standard' to quantify estrogens and their metabolites in serum and plasma due to improved specificity, high accuracy, and the ability to monitor multiple estrogens when compared with immunoassays. Ultra-high sensitivity can be obtained with pre-ionized derivatives when using triple quadruple mass spectrometers in the selected reaction monitoring (SRM) mode coupled with nanoflow LC. In this review, we have examined the special issues related to utilizing ultra-high sensitivity SID LC-SRM/MS-based methodology to accurately quantify estrogens and their metabolites in the serum and plasma from populations with low estrogen levels. The major issues that are discussed include: sample preparation for both unconjugated and conjugated estrogens, derivatization, chromatographic separation, matrix effects, and assay validation.