Estrogen Plus Progestin Doubles Risk of Venous Thrombosis

Charlene Laino

December 15, 2003

Dec. 15, 2003 (San Diego) — The use of estrogen plus progestin doubles the risk of venous thrombosis in women, according to a new analysis of data from the Women's Health Initiative.

"Body mass index and older age were the primary determinants of increased risk," Mary Cushman, MD, from the University of Vermont in Burlington, told a press briefing here at the 45th annual meeting of the American Society of Hematology.

Also, in a nested case control study, the researchers found that factor V Leiden was the primary genetic factor associated with increased thrombosis risk.

The primary analysis included 16,608 women aged 50 to 79 years who were randomized to receive either 0.625 mg per day of conjugated equine estrogen and 2.5 mg per day of medroxyprogesterone acetate, or placebo.

At about five years of follow-up, when the trial ended early after the risks of estrogen plus progestin were found to outweigh its potential benefits, the study showed that women in the estrogen plus progestin group had approximately a twofold increased risk of developing venous thrombosis compared with those receiving placebo.

At baseline, women in both groups were at similar risk for thrombosis, Dr. Cushman said. About one third of the women were aged 50 to 59 years,and 22% were aged 70 to 79 years; 84% were white; 35% had a body mass index (BMI) of 25 to 29; and one third had a BMI of 30 or higher.

Overall, 167 women in the estrogen plus progestin group developed thrombosis compared with 76 women in the placebo group. "This translated to a 2.1-fold increased risk, which was significant, with a 95% confidence interval [CI] of 1.6 to 2.7," Dr. Cushman said.

The risk was similar for deep vein thrombosis and pulmonary embolism, she added. Also, women appeared to be at greatest risk during their first year of taking estrogen plus progestin, when risk was increased fourfold, she said.

The nested case-control study included 147 women who developed thrombosis and 513 controls. Nearly 14% of cases tested positive for factor V Leiden compared with about 5% of controls, Dr. Cushman reported.

That translated to a 6.7-fold increased risk of thrombosis associated with factor V Leiden (95% CI, 3.1 - 14.5), she said.

"Factor V Leiden, inactivated at a rate approximately 10 times slower than normal factor V, increases a woman's risk for venous thrombosis," she said.

Benjamin Brenner, MD, associate professor of medicine at the Technion-Israel Institute of Technology in Haifa, and a specialist in thrombosis, said the findings "offer insights into which women are at increased risk of venous thrombosis."

Further studies are needed before physicians start testing for factor V Leiden prior to prescribing estrogen plus progestin, however, he said.

ASH 45th Annual Meeting: Abstract 719. Presented Dec. 9, 2003.

Reviewed by Gary D. Vogin, MD


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