From the Journals

Factors linked with increased VTE risk in COVID outpatients



Overall risk for venous thromboembolism (VTE) in nonhospitalized COVID-19 patients is low, but some of those patients may have factors that increase the risk and warrant more surveillance, according to a new retrospective cohort study.

Though VTE risk is well studied and significant in those hospitalized with COVID, little is known about the risk in the outpatient setting, said the authors of the new research published online in JAMA Network Open.

The study was conducted at two integrated health care delivery systems in northern and southern California. Data were gathered from the Kaiser Permanente Virtual Data Warehouse and electronic health records.

Nearly 400,000 patients studied

Researchers, led by Margaret Fang, MD, with the division of hospital medicine, University of California, San Francisco, identified 398,530 outpatients with COVID-19 from Jan. 1, 2020, through Jan. 31, 2021.

VTE risk was low overall for ambulatory COVID patients.

“It is a reassuring study,” Dr. Fang said in an interview.

The researchers found that the risk is highest in the first 30 days after COVID-19 diagnosis (unadjusted rate, 0.58; 95% confidence interval, 0.51-0.67 per 100 person-years vs. 0.09; 95% CI, 0.08-0.11 per 100 person-years after 30 days).

Factors linked with high VTE risk

They also found that several factors were linked with a higher risk of blood clots in the study population, including being at least 55 years old; being male; having a history of blood clots or thrombophilia; and a body mass index (BMI) of at least 30 kg/m2.

The authors write, “These findings may help identify subsets of patients with COVID-19 who could benefit from VTE preventive strategies and more intensive short-term surveillance.”

Are routine anticoagulants justified?

Previously, randomized clinical trials have found that hospitalized patients with moderate COVID-19 may benefit from therapeutically dosed heparin anticoagulants but that therapeutic anticoagulation had no net benefit – and perhaps could even harm – patients who were critically ill with COVID.

“[M]uch less is known about the optimal thromboprophylaxis strategy for people with milder presentations of COVID-19 who do not require hospitalization,” they write.

Mild COVID VTE risk similar to general population

The authors note that rates of blood clots linked with COVID-19 are not much higher than the average blood clot rate in the general population, which is about 0.1-0.2 per 100 person-years.

Therefore, the results don’t justify routine administration of anticoagulation given the costs, inconvenience, and bleeding risks, they acknowledge.

Dr. Fang told this publication that it’s hard to know what to tell patients, given the overall low VTE risk. She said their study wasn’t designed to advise when to give prophylaxis.

Physicians should inform patients of their higher risk

“We should tell our patients who fall into these risk categories that blood clot is a concern after the development of COVID, especially in those first 30 days. And some people might benefit from increased surveillance,” Dr. Fang said.

”I think this study would support ongoing studies that look at whether selected patients benefit from VTE prophylaxis, for example low-dose anticoagulants,” she said.

Dr. Fang said the subgroup factors they found increased risk of blood clots for all patients, not just COVID-19 patients. It’s not clear why factors such as being male may increase blood clot risk, though that is consistent with previous literature, but higher risk with higher BMI might be related to a combination of inflammation or decreased mobility, she said.


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