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Blood thinners still often used to treat patients with small lung blood clots

Kaiser Permanente study finds few patients qualify for surveillance and few physicians recommend it

When a patient arrives in the emergency department with a blood clot in their lungs, blood thinners are typically the first line of treatment. But over the years, questions have been raised about whether the risks associated with blood thinners outweigh the benefits of giving these drugs to patients who have a very small blood clot in a narrow lung artery.

David Vinson, MD

To address this concern, the American College of Chest Physicians (CHEST) released guidelines in 2016 that recommended certain low-risk patients undergo structured surveillance rather than receive blood thinners. But a new Kaiser Permanente study suggests that very few patients with these small blood clots are eligible for surveillance — and those who are eligible are almost always still treated with blood thinners.

“About 8 to 10 percent of patients with pulmonary embolism — blood clots in the lung — have clots lodged only in the subsegmental arteries, which are the narrowest arteries in the lungs,” said senior author David R. Vinson, MD, an adjunct investigator with the Division of Research and an emergency physician with The Permanente Medical Group. “We suspected that withholding blood thinners from select patients with these low-risk blood clots would be uncommon, because it went against how doctors are taught to treat patient with pulmonary embolism. We also thought it might be difficult for physicians to determine which patients qualified for surveillance. Our findings confirm this.”

The study, published August 2 in JAMA Network Open, used data collected from 21 Kaiser Permanente Northern California hospitals between 2017 and 2021. The research team looked at how many patients would be eligible for surveillance using conservative criteria they developed from the CHEST guidelines. They identified 666 patients who were diagnosed in the emergency department or outpatient clinics with small blood clots. Of these, 229 patients were considered lower risk and only 35 patients — 5% of the total study population — would have qualified for structured surveillance using the criteria modified from the CHEST guidelines.

If it’s not happening at Kaiser Permanente, it’s unlikely to be happening in hospitals that do not have this infrastructure.

— David Vinson, MD

Ultimately, only 1 of the 229 patients who were considered low risk was selected for structured surveillance. This suggests that among all patients with pulmonary embolism, surveillance without blood thinners would be an option for very few and have quite a limited role. The protocol for surveillance includes close clinical follow-up along with 2 lower-extremity ultrasounds to detect blood clots in the legs (deep vein thrombosis), which can potentially break away from the leg and float up to the lungs.

Mary Reed, DrPH

“Our study shows how difficult it can be to introduce new guidelines when the recommendations contradict long-standing ways of treating patients,” said study co-author Mary Reed, DrPH, a research scientist at the Division of Research.

“To look at whether surveillance could take a foothold in community practice, Kaiser Permanente Northern California was the perfect setting to study,” said Vinson, who chairs the Kaiser Permanente CREST Network, a multi-center collaborative for emergency medicine research. “Our treating physicians have ready access to specialty consultation, diagnostic imaging, and timely follow-up, all the ingredients needed for structured surveillance. If it’s not happening here, it’s unlikely to be happening in hospitals that do not have this infrastructure.”

Two large international clinical trials now underway are evaluating the use and safety of structured surveillance in this population of patients. “Until findings from those studies are published,” said Vinson, “we recommend these small blood clots be managed like their larger counterparts: treat with blood thinners, unless the patient’s risk for bleeding complications is high.”

The study was funded by the Kaiser Permanente Northern California Community Health Program and The Permanente Medical Group Delivery Science and Applied Research Program.

Co-authors include Samuel G. Rouleau, MD, of UC Davis Health; Mahesh J. Balasubramanian, MD, and Tad Antognini, MD, of The Permanente Medical Group, and Jie Huang, PhD, of the Division of Research.

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About the Kaiser Permanente Division of Research

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. For more information, visit or follow us @KPDOR.

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