Abstract P377: Oral Estrogen Hormone Therapy Use after Incident Venous Thrombosis and the Risk of Recurrence
Authors: Harrington Laura B , Wiggins Kerri L , Blondon Marc , Floyd James S , Bis Joshua C , Sitlani Colleen M , Rice Kenneth M , McKnight Barbara , Psaty Bruce M , Heckbert Susan R , Smith Nicholas L 
Published: July 2021

Background:
Oral estrogen hormone therapy (HT) is associated with a 2-fold increased risk of incident venous thrombosis (VT). Patterns of HT use following an incident VT and the association between HT use and recurrence are poorly characterized.

Objectives:
We aimed: (1) to characterize the use of oral estrogen HT in the 5 months following a first VT event; and (2) to evaluate whether the use of HT following an incident VT is associated with VT recurrence.

Methods:
In Group Health Cooperative (GHC), an integrated health care system, we identified an inception cohort of women aged 40-89 who experienced an incident VT from 2002 to 2010 (n=1,121). Using medical record review, we followed participants for VT recurrence, defined by physician diagnosis >2 weeks after the incident VT with clinical and/or imaging evidence of a new or expanded clot. HT use at 80% compliance was defined using GHC computerized pharmacy data. Characterization of HT use was restricted to women who survived for ≥5 months. Cox proportional hazards models were used to evaluate the association between time-varying current HT use and risk of recurrent VT, adjusting for race, baseline age, BMI, idiopathic incident VT, diabetes status, and the following time-varying covariates: cancer, CVD, and current anticoagulation use. In primary analyses, time was defined as days since incident event and censoring occurred at last follow-up or death. To evaluate the influence of possible HT misclassification soon after the incident event, we conducted 2 sensitivity analyses in which women entered the risk set after 30 and 60 days post-incident VT. Additional secondary Cox analyses were restricted to current HT users at the time of incident VT.

Results:
Of women who survived ≥5 months post-incident VT (n=993), 12% used oral estrogen HT at time of the incident event (n=123); among these, 90 (73%) discontinued use whereas 33 (27%) continued to fill ≥1 HT prescription within the next 5 months. Compared with users who stopped HT, persistent users were on average older (70 vs. 65 years), had a lower BMI (29 vs. 32 kg/m
2
), and were less likely to have experienced an incident PE as part of their first VT event (27% vs. 61%). Over an average follow-up of 3.4 years, we identified 164 recurrent VTs among 1,121 women. We found no evidence of an association between HT use after incident VT and VT recurrence risk, adjusted for confounders (HR=0.73; 95% CI: 0.26-1.99; p=0.53); no evidence of an association was found in sensitivity or secondary analyses.

Conclusions:
In this population of GHC enrollees, over one-quarter of oral estrogen HT users at VT incidence continued use and filled subsequent HT prescriptions in the 5 months following the incident event. We found no evidence of an association between post-event HT use and the risk of VT recurrence, however, the number of HT users in our population was small.