Practice of Epidemiology Reproducibility of Estradiol and Testosterone Levels in Postmenopausal Women Over 5 Years: Results From the Breakthrough Generations Study

Prospective cohort studies examining sex hormones in relation to cancer risk have generally collected blood sam-ples at 1 time point, with an assumption that hormone levels measured in these samples will be reliable markers of true levels at other times. In postmenopausal women, body fat is a major source of estradiol; therefore, changes in adiposity may affect the correlation of single measurements to more relevant long-term averages. To estimate the intraclass correlation coefficient (ICC) for estradiol and testosterone, we collected repeat blood samples from 119 postmenopausal women (average age = 59.4 (standard deviation, 4.7) years) from the United Kingdom during 2004–2005 and again during 2010–2011. The ICCs (adjusted for assay variation) were 0.73 (95 % confidence in-terval: 0.63, 0.82) for total estradiol and 0.59 (95 % confidence interval: 0.47, 0.72) for total testosterone. The ICCs were 3%–5 % larger after adjustment for change in body mass index (weight (kg)/height (m)2) or leptin, which are 2 markers of change in adiposity. There was no increase in ICCs after adjustment for change in age, alcohol con-sumption, smoking, exercise, time between waking and blood collection, or season. The results suggest that other factors account for within-woman variation in these sex hormones. estradiol; reproducibility of results; testosterone Abbreviations: BMI, body mass index; CI, confidence interval; CV, coefficient of variation; ICC, intraclass correlation coefficient; SHBG, sex-hormone binding globulin