Large Retrospective Observational Study: Blood Thinners Shouldn’t be Routinely Prescribed for COVID-19 Outpatient Cases

Large Retrospective Observational Study Blood Thinners Shouldn’t be Routinely Prescribed for COVID-19 Outpatient Cases

According to the U.S. Centers for Disease Control and Prevention (CDC), new COVID-19 weekly hospitalization has decreased from the national 7-day average peak in January 9, 2021 to an average of 4,984 admissions over this most recent established duration ending March 30, 2021. The corresponding period’s seven day new reported COVID-19 cases stands 62,167 meaning that the hospitalization rate at present is about 12% for all cases nationwide. Now a new retrospective cohort study sponsored by Kaiser Permanente’s Division of Research reveals that the vast majority of COVID-19 patients are at a low risk of developing blood clots and shouldn’t be routinely prescribed blood thinners. Based on the results from this recent Kaiser study, about 88% of COVID-19 cases, that is those outpatient cases, in most cases should not be routinely prescribed blood thinners. 

The Challenge

As reported recently by Kaiser Permanente’s Division of Research in Spotlight, the preventing of blood clots in COVID patients has been a concern since the start of the pandemic. “It was recognized within the first months of the pandemic that patients with COVID-19 were having more blood clots in the hospital and in the intensive care unit than other patients who were similarly ill,” said Nareg Roubinian, MD, an adjunct investigator at the Kaiser Permanente Northern California Division of Research and an intensive care doctor for The Permanente Medical Group. “This led us to reassess regional guidelines to prevent blood clots in hospitalized COVID-19 patients. At the same time, it was not clear what was the best treatment for outpatients with COVID.”

The Study

Published in JAMA Internal Medicine, this retrospective cohort study results were based on the analysis of 220,588 Kaiser Permanente Northern California (KPNC) members who were tested for COVID-19 from February 25 through August 31, 2020. In the 30 days after testing, 0.8% of the patients with a positive COVID-19 test developed a blood clot (venous thromboembolism) compared with 0.5% of the patients with a negative test result. 

Principal Investigator Point of View

Ashok Pai, MD, is a hematologist/oncologist working for The Permanente Group who also happened to be the study’s senior author. Dr. Pai went on the record, “Our study showed that the incidence of blood clots in outpatients who had a positive COVID-19 test and mild symptoms was similar to the incidence seen in patients who tested negative for the virus.”

A Key Question Answered

The new study helps answer that question. It found that in patients with COVID-19, blood clots were seen more often in those who were hospitalized than in those who were outpatients. It also confirmed findings from previous studies that found low rates of blood clots in COVID-19 patients after they left the hospital. “The incidence of blood clots in this group of patients is very low and no different than what we see in patients who are discharged from the hospital for other reasons,” said Pai.

Large blood clots can cause a heart attack, stroke, or blockage. Blood thinners are medications that help the blood flow smoothly and keep blood clots from forming or getting larger. Although they work well, they can also cause excessive bleeding. The Division of Research currently has a randomized phase 3 clinical trial underway looking at whether the drug thinner rivaroxaban (Xarelto) reduces the risk of blood clots, hospitalization, and death in outpatients with COVID-19.

“Generally blood thinners are low risk,” said Roubinian. “But because they can cause bleeding  you have to balance the benefit versus the risk, especially if you are considering using them to treat people with COVID-19 who are not very sick and may be at low risk for a blood clot. Our study suggests that we need to wait for the data from clinical trials to determine the best treatment approach and not routinely put all outpatients on blood thinners.”

Kaiser: A Major Research Player

The study was funded by a group within The Permanente Medical Group known as the Delivery and Applied Research Program (DARE). First TrialSite introduces The Permanente Medical Group and then the efforts of DARE.

The Permanente Medical Group is the largest medical group in the United States as well as one of the most distinguished. With 9,000 physicians and over 39,000 nurses and staff, the combined efforts help transform health care in a quest to deliver superior outcomes for over 4.5 million patients in Northern California. Now DARE is a research oriented group within the Permanente Medical Group. Cultivating research to drive evidence-based medicine, DARE facilitates working relationships between clinicians with high-impact research questions and investigators who have experience managing studies from start to finish.  

With a keen eye toward working to ensure gaps in knowledge and areas of improvement identified at the hospital level receive optimal support from dedicated research staff, the group leverages research networks within the organization, minimizing duplication of existing efforts, driving more urgent and efficient feasibility and cutting down on overall study startup costs.

About Kaiser Permanente 

The Kaiser Permanente Division of Research conducts, publishes and disseminates epidemiologic and health services research to improve the health and medical care of Kaiser Permanente members and society at large. It seeks to understand the determinants of illness and well-being, and to improve the quality and cost-effectiveness of health care. Currently, DOR’s 600-plus staff is working on more than 450 epidemiological and health services research projects. 

Lead Research/Investigators

Call to Action: Physicians may be interested in this recent study result. 


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  1. Are we supposed to trust JAMA again now? Or the CDC, which changes it’s mind every 4 days?

    So let get this straight: blood thinners prevent blood clots. Blood thinners are being used on hospitalized Covid patients. Hospitalized Covid patients are having blood clots, but not out-patients. Um, maybe it’s something else being done at the hospitals? Or perhaps they should try a milder blood thinner?