Salivary/Serum Progesterone Ratio Differs Between Menstrual Cycle Phases but Not Between Populations: Implications for Health, Reproductive, and Behavioral Research

Hypothesis 1 was supported. Consistent with prior reports for other populations, in these Bolivian women UF was higher and more variable in the follicular than in the luteal phase. The source(s) of phase-associated variation in UF deserves additional study, particularly the dynamic relationship to different conformers of corticosteroid-binding globulin (CBG). Hypothesis 2 was not supported. Paired P_Free-SAL and P_Total-VEN were highly correlated, and UF in these Bolivians was comparable to published values for other populations. Hypothesis 3 was not supported. There was no evidence that some individuals have consistently higher (or lower) UF than most other persons. In sum, these findings do not support the suggestions that the physiology underlying the relationship between P_Free-SAL and P_Total-VEN differs substantially and inexplicitly between populations and individuals. These results also reinforce the critical roles of fastidious attention to sample collection and handling, judicious assessment of assay results, and appropriate statistical methods when using ovarian steroid data in any project. We suggest some guidelines for meeting these requirements. Used with due consideration for its advantages and limitations, P_Free-SAL reliably tracks P_Total-VEN during the menstrual cycle and is a useful option in the biomarker toolkit. Just as it is costly to continue our work with tools not up to the task, so is it costly to discard useful tools without good reason. The development (and improvement through replication) of a robust toolkit for assessing changes in and the impacts of menstrual cycle hormones is foundational to reducing gender-based health disparities. (The linked file listed below under "Supporting Information" presents these findings in Spanish).