Towards the non-invasive determination of estradiol levels: Development and validation of an LC-MS/MS assay for quantification of salivary estradiol at sub-pg/mL level.

Background: Estradiol (E2) is a female sex hormone involved in several biological processes. Although E2 levels are commonly measured in blood samples, the use of non-invasive techniques (e.g. determination of salivary E2) would allow for the collection of repeated samples and the inclusion of a greater number of participants. Immunoassay-based techniques to measure salivary E2 failed to accurately mirror the variations observed in the plasmatic concentrations of E2 during the menstrual cycle probably due to the high sensitivity required (in the sub-pg/mL range). Therefore, sensitive and rugged analytical methods for the determination of salivary E2 are required. For this, we developed and validated an analytical methodology for the accurate determination of salivary E2.

Results: The method is based on chemical derivatization with 1,2-dimethyl-1H-imidazole-5-sulphonyl chloride and liquid chromatography-tandem mass spectrometry analysis by summing highly-specific SRM transitions. This strategy allowed for increasing the sensitivity of the method. The validation of the method showed an accurate and precise quantification of E2 in 1 mL of saliva even at 250 fg/mL (97 % accuracy and 15 % RSD intra-day, and 104 % accuracy and 18 % RSD inter-day). In order to evaluate its efficacy, we analysed saliva samples from 5 healthy female volunteers collected during a whole menstrual cycle. Our analyses showed that the variations in the concentration of E2 in the measured samples mirrored those expected during a complete menstrual cycle. Additionally, we validated the suitability of our method for determining salivary E2 levels during pregnancy.

Significance: To the best of our knowledge, this is the first method that allows to precisely and accurately measuring E2 in saliva samples along the whole menstrual cycle of healthy females. It is also suitable for the determination of estradiol during pregnancy. Its high sensitivity makes this strategy ideal for the evaluation of the role of hormone production in women's health.