What We Know About The Link Between Blood Clots And Covid-19

Patients with severe Covid-19 have “incredibly sticky blood”, increasing their risk of deep-vein thrombosis, say experts.

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Coronavirus has been linked to a higher risk of blood clots in those who become severely ill or require hospitalisation.

Multiple studies from different countries have noted a link between the two and now, experts in the UK are seeing it with their own eyes. Two of the most common types experts are seeing are deep-vein thrombosis (DVT) and pulmonary embolism (PE).

“We’re seeing a lot of [blood clot cases] with Covid-19,” Professor Beverley Hunt, a consultant in thrombosis, tells HuffPost UK. People who become severely ill with the virus – where they develop pneumonia and need hospitalisation – have “high rates” of blood clots, she says.

Prof Hunt, who is co-founder of charity Thrombosis UK, has also witnessed blood clots in patients who are at home and sick with Covid-19, although at lower rates.

So, why does it happen?

Firstly, what is deep-vein thrombosis and pulmonary embolism?

DVT is a blood clot that forms in a vein deep in the body, usually – but not always – the lower leg or thigh. The clot occurs when a person’s blood thickens and clumps together.

Symptoms of DVT in the leg include: throbbing or cramping pain in one leg (rarely both legs), swelling in one leg, warm skin around the painful area, red or darkened skin around the painful area, and swollen veins that are sore to touch.

If a blood clot in a deep vein breaks off and travels through the bloodstream, it becomes an ‘embolus’ and can travel to an artery in the lungs and block blood flow, which is known as pulmonary embolism (PE). This causes symptoms such as a cough, shortness of breath, chest pain, or in the worst case scenario, it can block all blood supply to the lungs and prove deadly.

DVT is often associated with people who sit for prolonged periods of time, such as those who travel on long-haul flights – although Prof Hunt says getting a clot from a flight is rare. “The number one risk factor is going into hospital or being unwell,” she says.

At the start of lockdown, haematologists anticipated there might be a rise in DVT due to an increase in the number of people sitting for prolonged periods of time and not leaving their homes. What they didn’t expect was that the virus itself would be the cause of most cases they were seeing in hospital.

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What we know about blood clots and Covid-19

So far, it’s unclear whether blood clots are a direct consequence of being ill from Covid-19 or a complication of the inflammation that occurs in the body when you get it.

Among patients with coronavirus admitted to intensive care units (ICU) in the Netherlands, 31% developed blood clots in the deep veins of the legs or in the lungs, which researchers said was “remarkably high”. In Lombardia, Italy, a screening of 108 Covid-19 patients revealed almost a quarter (23%) of patients on ventilators in ICU had DVT and 20% had PE.

In the UK, a study from Brighton and Sussex Medical School (BSMS) found that in 274 people with Covid-19 admitted to hospital, 7.7% were diagnosed with a blood clot. The most common type of blood clot, seen in 76.2% of these cases, was PE.

Lead author Dr Chi Eziefula, senior lecturer in infection at BSMS, said identifying Covid patients at risk of developing blood clots is important for two reasons. Firstly, because blood clots are linked to a risk of death and secondly because they can be treatable with blood thinning medications.

Professor Mark Whiteley, a leading vein expert from The Whiteley Clinic, says the increased risk of blood clots seems to occur in people who are unwell with Covid-19 and who have raised blood markers for clotting, called D-dimers.

However, as Thrombosis UK explains: “Although D-dimer levels are elevated in most patients with blood clots, D-dimer levels also are elevated in many other disorders including infection. Therefore, an elevated D-dimer level in patients with Covid-19 infection alone is not evidence that they have a clot.”

Prof Hunt says there are three key issues at play when diagnosing blood clots: “Is the blood stickier? Is the patient immobile so that there’s reduced blood flow in the legs? And have there been any changes to the lining of the patient’s blood vessels?”

In some severely ill coronavirus patients, all of these things are happening, says Hunt. Patients with severe Covid-19 have “incredibly sticky blood”, up to levels she’s never seen before. “And, of course, when you’re not very well, you don’t move around as much, you’ve got reduced [blood] flow,” she says.

“So it’s the perfect recipe for deep-vein thrombosis and pulmonary embolism.”

In hospital settings, it’s been suggested that some patients with blood clots might benefit from blood thinning drugs to reduce the risk of clots. “They are suggesting it might be sensible to do this on all patients admitted with Covid-19 who are unwell, to see if this helps,” says Prof Whiteley.

It’s believed NHS England is preparing guidance for hospitals on issuing higher doses of blood-thinning medication to Covid-19 patients, according to The Telegraph.

What can you do?

The key thing anyone can do is keep moving, says Prof Hunt, even when self-isolating with suspected coronavirus symptoms.

When you walk, the muscles in your legs are squeezing the veins and pushing the blood up the leg. But if you sit still even for 90 minutes, the blood flow stagnates in the leg and the flow rate falls behind the knee by about 50%, says Prof Hunt.

“We know that people who work for very long periods in the office or at home, who don’t get up and walk around, have a higher rate of DVT,” she says.

Hunt urges people – both those who feel healthy and those who suspect they might have coronavirus – to move about every 60 to 90 minutes, and drink plenty of water. “Dehydration is another risk factor for DVT, so people need to keep drinking during the day,” she says.

The advice is particularly important for groups more at risk of developing DVT, such as people with obesity, as well as women on the combined oral contraceptive pill, women taking HRT in tablet form and people who have cancer.