Results Summary

What was the research about?

After the fracture of certain bones, like the hip, thigh, or the long bones in the arms or legs, people have a high risk of getting blood clots. Blood clots form in the veins of the arms and legs and can be fatal if they travel to the lungs. A blood thinner called low molecular weight heparin, or LMWH, can help prevent clots. LMWH is injected under the skin. Aspirin is another medicine that can prevent clots; it is taken by mouth and is an inexpensive medicine.

In this study, the research team compared aspirin versus LMWH to prevent blood clots after treatment for a fracture in the hip, thigh, or the long bones in the arms and legs. The team wanted to learn if aspirin wasn’t any worse than LMWH in preventing deaths after treatment for a fracture.

What were the results?

At three months, the group of patients who received LMWH and the group who received aspirin didn’t differ in:

  • The percentage of patients who died for any reason
  • The percentage of patients who had blood clots in the lungs, bleeding, infections, or wound issues
  • Patient out-of-pocket costs

Compared with the group of patients who were prescribed LMWH, the group of patients who were prescribed aspirin was more likely to be satisfied with and continue taking their medicine. But they had slightly higher rates of blood clots in the leg. It is unclear if this difference in blood clot rates—1.7 percent versus 2.5 percent—is clinically meaningful.

Who was in the study?

The study included 12,211 adults receiving treatment for fractures at one of 21 trauma centers in the United States. The average age was 45, and 62 percent were men.

What did the research team do?

The research team assigned patients by chance to receive a prescription for either LMWH or aspirin. Patients took LMWH by injection twice a day. Patients took aspirin by mouth twice a day. Doctors decided how long patients should be on each medicine.

After three months, the research team reviewed health records for deaths and other health outcomes. The team interviewed patients about how satisfied they were with their treatment. They also tracked how often patients took their medicine and their out-of-pocket costs.

What were the limits of the study?

All patients could receive up to two doses of LMWH before starting the study. Results may have differed if all patients had only received either aspirin or LMWH.

Future studies could compare aspirin with other blood thinners for preventing blood clots after a fracture.

How can people use the results?

Patients and doctors can use the results when considering medicines to prevent blood clots after a fracture in the hip, thigh, or the long bones in the arm or legs.

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 report did not include some of the statistical analyses proposed in the original study protocol, specifically non-inferiority tests of non-fatal pulmonary embolism or deep-vein thrombosis and superiority tests of rate of complications. The researchers explained that their statistical analysis plan evolved and recommended that the reviewers compare the final report to the final study protocol and statistical analysis plan found in the report’s appendix.
  • The reviewers also noted that the report did not include the two planned interim analyses and asked for those to be added since the interim analyses could have led the data and safety monitoring board to end the study early. The researchers added a summary of the interim analyses, which were reviewed only by the data and safety monitoring board and resulted in no study protocol changes.
  • Reviewers requested more rationale for comparing aspirin to low-molecular-weight heparin rather than other oral anticoagulants for thromboprophylaxis in orthopedic trauma patients. The researchers explained they wanted to focus on treatments currently in clinical use.

Conflict of Interest Disclosures

Project Information

Robert V. O'Toole, MD
University of Maryland Baltimore
A Randomized Pragmatic Trial Comparing the Complications and Safety of Blood Clot Prevention Medicines Used in Orthopedic Trauma Patients

Key Dates

July 2016
May 2023

Study Registration Information


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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
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Last updated: November 14, 2023