Does the route of administration for estrogen hormone therapy impact the risk of venous thromboembolism? Estradiol transdermal system versus oral estrogen-only hormone therapy.

OBJECTIVE The aim of this study was to quantify the magnitude of risk reduction for venous thromboembolism events associated with an estradiol transdermal system relative to oral estrogen-only hormone therapy agents. METHODS A claims analysis was conducted using the Thomson Reuters MarketScan database from January 2002 to October 2009. Participants 35 years or older who were newly using an estradiol transdermal system or an oral estrogen-only hormone therapy with two or more dispensings were analyzed. Venous thromboembolism was defined as one or more diagnosis codes for deep vein thrombosis or pulmonary embolism. Cohorts of estradiol transdermal system and oral estrogen-only hormone therapy were matched 1:1 based on both exact factor and propensity score matching, and an incidence rate ratio was used to compare the rates of venous thromboembolism between the matched cohorts. Remaining baseline imbalances from matching were included as covariates in multivariate adjustments. RESULTS Among the matched estradiol transdermal system and oral estrogen-only hormone therapy users (27,018 women in each group), the mean age of the cohorts was 48.9 years; in each cohort, 6,044 (22.4%) and 1,788 (6.6%) participants had a hysterectomy and an oophorectomy at baseline, respectively. A total of 115 estradiol transdermal system users developed venous thromboembolism, compared with 164 women in the estrogen-only hormone therapy cohort (unadjusted incidence rate ratio, 0.72; 95% CI, 0.57-0.91; P = 0.006). After adjustment for confounding factors, the incidence of venous thromboembolism remained significantly lower for estradiol transdermal system users than for estrogen-only hormone therapy users. CONCLUSIONS This large population-based study suggests that participants receiving an estradiol transdermal system have a significantly lower incidence of venous thromboembolism than do participants receiving oral estrogen-only hormone therapy.