Long Covid sufferers at far greater risk of blood clots, study says

Long Covid sufferers are at four times greater risk of abnormal blood clotting, according to a new study.

Researchers found that blood abnormality was much more likely in those experiencing difficulties with basic exercise more than 12 weeks after their coronavirus infection.

The study, published in the journal Blood Advances, is the first to link blood clotting with reduced exercise capacity in people with long Covid – also known as Post-Covid Syndrome – a condition that occurs in people previously infected with the virus.

Symptoms can last months after the initial infection and include fatigue, shortness of breath and brain fog. One study estimated that up to half of all people who recover from the virus experience lingering symptoms.

Study author Dr Nithya Prasannan, of University College London Hospital, said: “By definition, this syndrome occurs when one experiences Covid-related symptoms long after the onset of infection that we can’t attribute to any other cause or diagnosis.”

“This study offers us laboratory and clinical evidence to begin to understand why some people experience long Covid symptoms,” Dr Prasannan said.

Team lead Dr Melissa Heightman assessed people in an outpatient post-Covid clinic between July 2020 and May 2021 to conduct the study.

Participants were said to have long Covid if they experienced symptoms three months after the onset of their original infection which persisted for at least a further two months and in the absence of other contributing diagnoses.

Researchers measured markers of abnormal blood clotting by assessing the relative levels of two proteins. They analysed the ratio of Von Willebrand factor (VWF), a protein important in blood clotting, and ADAMTS13, a protein that cuts or splices VWF to prevent it from clogging blood vessels.

If there was significantly more VWF than ADAMTS13 in the bloodstream, scientists characterised patients as being in a prothrombotic state – meaning they could face a greater risk of developing blood clots.

The researchers also made participants do exercise tests, such as walking on a flat surface and changing from sitting to standing, all while wearing oxygen monitors. Their blood was tested before and after the activities so the scientists could measure their lactate levels and determine how their body responds to exercise.

During exercise, the body converts sugar into energy using oxygen. When there is not enough oxygen the body produces lactate instead, which can be turned into energy without oxygen.

In the study, participants who had low oxygen levels, measured by a sensor on their finger, while exercising and/or a rise in lactate afterwards were said to have an impaired exercise capacity. Notably, patients with raised levels of blood clotting markers were four times more likely to have an impaired exercise capacity.

The research team plan to use a variety of methods to measure how a patient’s risk of thrombosis can change over the course of their long Covid illness.

Dr Prasannan said the study could help not only with identifying the mechanical causes of long Covid but may also offer insight into potential treatments.

“I encourage people experiencing long Covid to participate in clinical trials when available because the more data we have, the better we can understand this condition.”


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