The Comparative Safety of Direct Oral Anticoagulants Versus Warfarin for the Treatment of Venous Thromboembolism

The objective of this study is to determine whether the use of direct oral anticoagulants (DOACs) is associated with an increased risk of major bleeding compared to warfarin use, for the treatment of venous thromboembolism (VTE).

A common-protocol approach will be used to conduct retrospective cohort studies using administrative health care data from eight jurisdictions (the Canadian provinces of Alberta, Manitoba, Nova Scotia, Ontario, Quebec, Saskatchewan, as well as the United Kingdon (UK) Clinical Practice Research Datalink and the United States (US) Marketscan). Briefly, the Canadian databases include population-level data on physician billing, diagnoses and procedures from hospital discharge abstracts, and dispensations for prescription drugs. The data in Nova Scotia, Ontario, and Alberta will be restricted to patients aged 65 years and older, as prescription data are not available for younger patients. The CPRD is a clinical database that is representative of the UK population and contains the records for patients seen at over 680 general practitioner practices in the UK.

In each jurisdiction, the investigators will assemble a study cohort that includes all patients newly prescribed a DOAC or warfarin from January 1, 2009 (or the earliest date of data availability at each site) to October 2, 2014 (or 180 days before the last date of data availability at each site, if earlier), that had an incident diagnosis of VTE within the 30 days prior to the date of the prescription. The date of study cohort entry will be defined by the prescription (for CPRD) or dispensation (for all other sites) date of the newly prescribed DOAC or warfarin. Patients in the study cohort will be followed from the date of study cohort entry until an event (defined below), censoring due to death or departure from the database, 3 months after cohort entry, or the end of the study period (March 31, 2015), whichever occurs first.

Exposure to a DOAC will be defined as a new prescription for a DOAC on the date of cohort entry. Exposure to warfarin will be defined as a new prescription for warfarin on the date of cohort entry. The investigators will use an analysis analogous to an intention-to-treat approach. The primary outcome will be defined as a hospitalization or emergency department visit for a major bleed within 3 months of cohort entry.

The study cohort will be analyzed using a matched cohort design, where up to 5 warfarin users will be matched to each DOAC user on sex, age, cohort entry date, and propensity score (which will be constructed using a multivariable logistic regression model estimating the odds of being treated with DOACs, while adjusting for a number of pre-identified covariates to account for baseline difference at the time of cohort entry). The hazard of major bleeding with DOAC use vs. warfarin use will be estimated using Cox-proportional hazards regression models accounting for the potential correlation in the matched pairs. Meta-analyses of the site-specific results will then be performed using fixed or random effects models (according to results of Chi-squared tests for heterogeneity). Sensitivity analyses will be conducted, all defined a priori, to assess the robustness of the results.