Study Suggests No Added Risk of Blood Clots in COVID-19 Outpatients

Jaleesa Baulkman

April 06, 2021

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

The incidence of venous thromboembolism (VTE) in nonhospitalized patients with COVID-19 was not significantly different from patients without the infectious disease, according to a new study published in JAMA Internal Medicine.

National Institutes of Health guidelines recommend blood thinners to prevent blood clots in patients hospitalized with COVID-19. However, the new study provides more insight on the best treatment approach for COVID-19 outpatients.

"[COVID-19's] rapid global progression and impact has caused us to make and modify treatment decisions at a pace that we never have in modern medicine," study author Nareg Roubinian, MD, an investigator at Kaiser Permanente, Oakland, Calif., said in an interview.

"As with other potential therapies for COVID-19, blood thinners need to be prospectively studied in a clinical trial to determine if they improve patient outcomes," Roubinian added.

The increased risk of blood clots in patients hospitalized with COVID-19 has been a major issue throughout the pandemic. In fact, one study published in November 2020 found that more than half of patients hospitalized with the illness have prothrombotic antiphospholipid (aPL) autoantibodies in their blood, which could contribute to venous and arterial thromboembolism.

Although it was clear many hospitalized patients diagnosed with COVID-19 were developing more clots, researchers of the current study were not sure if this trend would also be seen in outpatients.

"Most people with COVID-19 do not need to be hospitalized, and we needed to know how often patients outside the hospital were having blood clots," said Roubinian.

For the study, Roubinian and colleagues examined data on 220,588 patients who were members of Kaiser Permanente Northern California health plan and were tested for COVID-19 between Feb. 25 and Aug. 31, 2020. They then reported on the 30-day incidence of outpatient and hospital-associated blood clots following the COVID-19 diagnosis. Patients who were asymptomatic at the time of testing or had received anticoagulants within the last year were excluded.

"We knew from other studies that patients with COVID-19 often get sicker in the first few weeks after infection. What we didn't know was whether COVID-19 patients were developing blood clots but not pneumonia or were developing blood clots at the same time as they developed pneumonia," said Roubinian, an intensive care doctor with the Permanente Medical Group in Oakland, Calif. "Following the patients for 30 days allowed us to focus on the time period from infection to when blood clots were most likely to develop."

Researchers found that of the cohort who took the COVID-19 test, 11.8% had a positive result. Within 30 days of the COVID-19 test, 0.8% of patients with a positive result were diagnosed with VTE compared to 0.5% of those who received a negative test result. They also found that viral testing took place in an outpatient setting for 59.1% of the patients with a positive viral test who later developed VTE. Of those patients, 76.1% had to be hospitalized.

Roubinian said he was surprised to see that the blood clotting in outpatients with COVID-19 was similar in frequency to what he saw in patients without the infection.

"Our findings suggest that blood clots do occur in COVID-19 patients but not on a scale where we need to put all or many COVID outpatients on blood thinners," he said. "As with other potential therapies for COVID-19, blood thinners need to be prospectively studied in a clinical trial to determine if they improve patient outcomes."

In December 2020, three trials investigating the risk and benefits of increased levels of anticoagulation in hospitalized COVID-19 patients were paused because of safety issues. The trials would have enrolled critically ill COVID-19 patients for whom therapeutic doses of anticoagulation drugs showed no benefit.

Anticoagulants are associated with bleeding risks, including prolonged nosebleeds and vomiting or coughing up blood.

Instead of prescribing the routine use of thromboprophylactic drugs to COVID-19 outpatients, Roubinian believes it would be helpful to learn how to determine whether a patient at risk of becoming sick or being hospitalized would benefit from being treated with such drugs.

Roubinian reported receiving grants from the National Institutes of Health and the National Heart, Lung, and Blood Institute during the conduct of the study.

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