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Mathematical methods exist to determine the fractions of sex hormones bound to albumin, bound to sex hormone binding globulin (SHBG), or unbound, using total hormone concentration and SHBG concentration. We used data from eight prospective studies of postmenopausal women to assess the validity of these estimates for fractions of estradiol (E2) and to investigate the impact of using calculated values in breast cancer relative risk (RR) models. Comparisons were made between measured and calculated concentrations of free and non-SHBG-bound E2 in four studies. Relationships between the hormone fractions were investigated and a sensitivity analysis of the calculation performed. Breast cancer RRs were estimated using conditional logistic regression by quintiles of free E2. There is a high correlation (r > 0.91) between calculated and measured values of both free and non-SHBG-bound E2. The calculation is highly sensitive to total hormone concentration but is relatively insensitive to SHBG concentration. In studies with both measured and calculated values, the RRs of breast cancer by quintile of free E2 were almost identical for both estimates; using calculated values in all possible studies the RR in the highest compared with the lowest quintile of free E2 was 2.29 (95% confidence interval, 1.65–3.19). The mathematical method used to calculate fractions of E2 is valid, and RR analyses using calculated values produce similar results to those using measured values. This suggests that for epidemiological studies, it is only necessary to measure total E2 concentration and SHBG concentration, with hormone fractions being obtained by calculation, producing savings in cost, time, and serum.
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