Blood clots increased among post-menopausal women who used oral estrogens compared to those who used transdermal estrogen, in a French study presented at the American Heart Association's Arteriosclerosis, Thrombosis and Vascular Biology Annual Conference 2010.

Among the 60 women who were treated by transdermal estrogen, 23 (38 percent) used a patch and 37 (62 percent) a gel.

The researchers used data from healthy women to study hormone therapy's impact on the generation of thrombin, the key enzyme in clot formation and an emerging surrogate marker of venous thromboembolism.

For the analysis researchers used a random group of 533 postmenopausal women without factor V Leiden mutation and G20210A prothrombin mutation. Of the 533, nine were older than 80 years, 20 were missing data regarding hormone therapy use and two women used a hormone therapy other than oral or transdermal estrogens.

The team measured endogenous thrombin potential (ETP) in stored blood samples of the remaining 502 post-menopausal women without factor V Leiden mutation and G20210A prothrombin mutation, aged 65 to 80 years old. They were classified according to their use of hormone therapy: non-users (421), users of oral estrogens (21) and users of transdermal estrogens (60).

After adjusting for confounders, researchers found:

• Higher average levels of ETP among women using oral estrogens compared with non-users; there was no such difference among those using transdermal estrogens.
• Results were similar among women using estrogens alone or combined with progestogens.
• Significant difference in thrombin generation between women using oral and transdermal estrogens.

The findings add to epidemiological evidence that the route of estrogen administration is an important determinant of the venous thromboembolism risk among women using hormone therapy, researchers said.