Results Summary

What was the research about?

Each year, more than 500,000 people in the United States are hospitalized for blood clots that form in their veins. Many people who have a clot will have another within 10 years. Patients can take medicines called blood thinners to treat and prevent new clots. But blood thinners may cause problems, like severe bleeding. Questions remain about how long people should take these medicines to prevent future clots.

In this study, the research team compared ways to prevent blood clots in people who have already had one. The team compared patients who took blood thinners after a clot for 3 months with patients who took them for more than 3 months and up to 12 months. The team also compared the safety of taking three common blood thinners—warfarin, apixaban, and rivaroxaban—for more than 3 and up to 12 months.

The research team looked at risk of a hospital stay for new blood clots or bleeding.

What were the results?

Compared with patients who stopped taking blood thinners after three months, patients who took blood thinners for more than three months had:

  • A lower risk of a hospital stay to treat new blood clots within six months
  • A higher risk of a hospital stay to treat severe bleeding within six months

Statistical tests found that, among patients who took blood thinners for more than three months, patients taking:

  • Apixaban had a lower risk of a hospital stay to treat new blood clots than patients taking warfarin
  • Apixaban and patients taking rivaroxaban had a similar risk of a hospital stay to treat new blood clots
  • Rivaroxaban and patients taking warfarin had a similar risk of a hospital stay to treat new blood clots

The risk of a hospital stay to treat severe bleeding didn’t differ among patients who took warfarin, apixaban, or rivaroxaban.

Who was in the study?

The study included health records from three nationwide health insurance claims databases for 90,921 adult patients. All patients had been in the hospital for a blood clot and took blood thinners after leaving the hospital for at least 90 days. The average age was 70, and 57 percent were women.

What did the research team do?

The research team looked at health records from 2010 to 2021 to find patients who had been hospitalized for a blood clot and who hadn’t used blood thinners before the clot. Then the team tracked hospital stays for patients who filled prescriptions for a blood thinner and took it for at least 90 days.

Patients with prior blood clots and doctors helped design the study.

What were the limits of the study?

The study didn’t assign patients by chance to length of treatment. Differences between patients who extended treatment and who stopped treatment may have affected study results.

Future research could determine the best length of time for patients to take blood thinners to reduce the risk of future blood clots and bleeding.

How can people use the results?

Patients and doctors can use these results when considering treatments for blood clots.

PCORI identified long-term use of blood thinner medicines as an important research topic. Blood thinners help prevent blood clots but may cause health problems if taken for a long time. Patients, clinicians, and others wanted to learn: How do different long-term blood thinner treatments compare for patients who have had a blood clot? To help answer this question, PCORI launched an initiative in 2015 on New Oral Anticoagulants (NOACs) in the Extended Treatment of Venous Thromboembolic Disease. The initiative funded this research project and others.

Final Research Report

View this project's final research report.

Peer-Review Summary

Peer review of PCORI-funded research helps make sure the report presents complete, balanced, and useful information about the research. It also assesses how the project addressed PCORI’s Methodology Standards. During peer review, experts read a draft report of the research and provide comments about the report. These experts may include a scientist focused on the research topic, a specialist in research methods, a patient or caregiver, and a healthcare professional. These reviewers cannot have conflicts of interest with the study.

The peer reviewers point out where the draft report may need revision. For example, they may suggest ways to improve descriptions of the conduct of the study or to clarify the connection between results and conclusions. Sometimes, awardees revise their draft reports twice or more to address all of the reviewers’ comments.  

Peer reviewers commented and the researchers made changes or provided responses. Those comments and responses included the following:

  • The reviewers noted that the researchers used a composite outcome consisting of recurrent venous thromboembolism, major bleeding, and all-cause mortality for all of the data sets they reviewed even though the mortality component is only available in one of the data sets. The reviewers asked the researchers to explain how this was handled for the other two data sets. The researchers explained that mortality in the two other data sets was captured for in-patient deaths and for outpatient could be inferred from insurance disenrollment. The researchers noted in their study limitations section that the results for this composite outcome should be interpreted with caution given the limitations in capturing mortality.
  • One reviewer questioned whether the study’s comparisons of treatment continuation versus treatment discontinuation both included only data for those patients who were still alive at each time point, noting lack of that data restriction could introduce bias to the results. The researchers responded by including additional information in their methods specifying that all patients were followed unless they disenrolled from insurance, there were no further data on them, or they died.
  • The reviewers requested that the researchers expand their discussion to consider potential confounding, lack of focus on race and ethnicity, and lack of discussion regarding clinical implications of the study results. The researchers expanded their discussion section as suggested, including some reorganization with subheadings so that readers could follow these points more clearly.

Conflict of Interest Disclosures

Project Information

Katsiaryna Bykov, PharmD, ScD
Brigham and Women's Hospital
The Dabigatran, Apixaban, Rivaroxaban, Edoxaban, Warfarin Comparative Effectiveness Research Study (The DARE warfarin CER study)

Key Dates

July 2016
November 2023

Study Registration Information


Has Results
Award Type
Health Conditions Health Conditions These are the broad terms we use to categorize our funded research studies; specific diseases or conditions are included within the appropriate larger category. Note: not all of our funded projects focus on a single disease or condition; some touch on multiple diseases or conditions, research methods, or broader health system interventions. Such projects won’t be listed by a primary disease/condition and so won’t appear if you use this filter tool to find them. View Glossary
Populations Populations PCORI is interested in research that seeks to better understand how different clinical and health system options work for different people. These populations are frequently studied in our portfolio or identified as being of interest by our stakeholders. View Glossary
Intervention Strategy Intervention Strategies PCORI funds comparative clinical effectiveness research (CER) studies that compare two or more options or approaches to health care, or that compare different ways of delivering or receiving care. View Glossary
State State The state where the project originates, or where the primary institution or organization is located. View Glossary
Last updated: November 14, 2023