Watch out for blood clots: Covid can increase your risk

DVTs tend to form in the deep veins of the leg and the most common symptom is a painful and occasionally swollen calf all too often attributed to a pulled muscle. The main risk from delayed diagnosis is that part of the clot will break off and travel to the lungs, causing a PE. And the risk is considerable: about a third of patients with large DVTs extending up into their thighs develop a PE, and one in three will die.

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Well-established risk factors for DVT/PE include having had one before and/or a strong family history (some people have “sticky” blood due to inherited genes), being over 60, carrying too much weight, smoking, recent surgery/trauma, immobility (including on long-haul flights of more than four hours), pregnancy and taking some types of HRT and contraception. As well as any serious underlying illness, including cancer, that triggers inflammation, clotting abnormalities and/or immobilises you in bed — at home or in hospital.

Covid can still require hospital admission, but that is much less likely thanks to vaccination and most of us having caught it at least once. However, it can still increase the risk of a DVT or PE even if you don’t become that ill and, at least in this respect, it differs from most other viral coughs and colds.

We know that Covid can induce abnormal clotting that clogs the blood supply to the lungs and other organs, including the brain. Indeed, vascular “sludging” has recently been put forward as a possible cause of the mental fogging often seen in long Covid. And a large Swedish study has highlighted a worrying link with DVT and PE too.

Researchers compared DVT/PE rates in a million people who had tested positive for Covid — some who had been very ill, many who had not — with four million who had not (knowingly) had the virus. Over the six months after testing positive the DVT rate in the group with Covid was four times higher than those who had not had it (1 in 250 versus 1 in 1,000). And for PE the risk was ten times higher in the Covid group at about 1 in 600.

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Three caveats. First, the Swedish study was an observational one so it can’t prove Covid was responsible, it can only identify a link. Second, DVT/PE is still comparatively rare after Covid. And third, the study looked at the first year of the pandemic, so most people were unvaccinated. However, given all the other evidence that Covid induces abnormal clotting, it doesn’t require a giant leap of faith to assume it was responsible.

More research is needed to study the impact of vaccines on DVT/PE rates, but the Swedish study did suggest that the more severe your symptoms the greater the risk of a blood clot. And we know that vaccines do mean any symptoms are likely to be milder.

Whatever the cause of your DVT/PE, prompt diagnosis is key. Diagnosing a DVT in someone exhibiting all the classic signs isn’t hard and, once suspected, can be clinched using blood tests and/or scans. However, most DVTs are not that obvious. As many as 80 per cent are silent until a bit breaks off and becomes lodged in the lungs.

Even then diagnosis can be missed. The textbook symptoms of a PE are sudden-onset chest pain made worse by breathing deeply, breathlessness and coughing up blood, but smaller PEs often present as nonspecific unexplained shortness of breath.

Once picked up, prompt treatment with anticoagulants (typically a once or twice-daily pill) can stabilise the clot and reduce the risk of PE, but these need to be continued for at least three months and sometimes for life.

So please be aware of the symptoms of blood clots and report them. And if you have a DVT and there is no obvious trigger, particularly if you are over 40, ask your GP about further investigations, including looking for an underlying cancer (such as prostate or breast). And, finally, keep up to date with your Covid boosters, and avoid anyone who might have it. Covid, however minor, isn’t just a cold.

For more details on the diagnosis, management and prevention of DVT visit thrombosisuk.org

Blood clots: the facts

● In a typical (pre-Covid) year about 5,000 people die every year in England alone from blood clots, most of them acquired while in hospital — following trauma, serious illness or for surgery — with half only diagnosed once they are back home.

● People over 60 are most at risk but they can occur at any age. Every year at least 2,500 people under 40 in England and Wales develop a DVT, 250 of whom will die.

● The risk varies depending on the individual and why they are in hospital but as many as one in three surgical patients will develop a DVT unless specific preventive measures are taken (these should now be routine).

● If you are admitted to hospital, ask the doctor or nurse what steps will be taken to reduce the risk of DVT. All patients should be individually assessed so that preventive measures such as anticoagulants can be tailored accordingly.