By Rebecca Smith, Medical Editor
Published: 7:15AM GMT 04 Dec 2009
Lying in a hospital bed for extended periods after surgery increases the risk of a clot because it restricts blood circulation around the body.
Most people at risk are given drugs to thin their blood, but it was thought that for most patients, the heightened risk lasted only a few days - and reduced when they were up and moving around normally.
But now research has shown that surgery patients are still at a higher risk of a clot for seven to 12 weeks after surgery.
The findings were described as a 'wake-up call' to surgeons to prescribe blood-thinning drugs more often and for longer.
Around 25,000 hospital patients in Britain die after developing blood clots, also known as deep-vein thrombosis, and it is thought around 70 per cent of these could be prevented by more widespread use of anti-clotting drugs.
In August last year Sir Liam Donaldson, chief medical officer, recommended all hospital inpatients be assessed for blood clot risk and put those at risk on medication.
The new study published online in the British Medical Journal found hip and knee replacements, as well as surgery for cancer, make people particularly vulnerable to clots.
The study - on more than 947,000 British women - said that patients were 70 times more likely to be admitted to hospital with a blood clot in the first six weeks after surgery than someone who had not had an operation.
It had been thought patients in hospital were at eight times the risk of having a clot.
The risk was highest in the third week after an operation, said the experts, who were led by a team from the University of Oxford.
It was estimated that one in 140 middle-aged women undergoing surgery as an inpatient in Britain will be admitted with a blood clot in the 12 weeks after surgery.
That rises to one in 45 after a hip or knee replacement and one in 85 after surgery for cancer.
The risk of blood clots reduced over time but was still evident between seven and 12 weeks after surgery.
Overall during the six-years of follow-up, 5,419 women were admitted to hospital with pulmonary embolism or deep vein thrombosis and 270 died from their blood clot.
The authors concluded: "These findings suggest that the risk of venous thromboembolism (blood clot) after surgery is greater and lasts for longer than previously thought."
In an accompanying editorial, Dr Alexander Cohen, a vascular physician from King's College Hospital in London, said the research acted as a 'wake-up call to all surgeons'.
He said guidance on giving patients anti-clotting drugs after surgery varies and is limited to 'four weeks for some orthopaedic patients with hip fractures or hip replacements who have additional risk factors, or five weeks for patients having hip surgery and high risk general surgery'.
Patients admitted as day-cases are usually not given anti-clotting drugs at all.
Dr Cohen said it was likely the study underestimated the actual number of blood clots.
He said only six in ten patients worldwide who have surgery as an inpatient receive drugs to thin their blood to prevent clots and this varies greatly between countries.
Dr Tim Chico, Senior Clinical Lecturer and honorary Consultant Cardiologist, at University of Sheffield, said: " It is crucial to recognise risk of DVT after surgery, and we need to improve its prevention.
"I hope this research helps patients understand the need for things like compression stockings and blood thinning injections. DVT doesn't only strike people undergoing surgery, so it is also important to avoid obesity and other risk factors.”
Dr Trevor Baglin, Consultant Haematologist at Addenbrookes Hospital and Chairman of the British Committee for Standards in Haematology, said: “This is an extremely timely and informative publication. It comes following the Chief Medical Officer’s stated desire that all patients admitted to hospital should be assessed for risk of hospital-acquired thrombosis.
“At the moment risk assessment for hospital acquired thrombosis is not mandated and interestingly many of the highest ranking hospitals in the recently reported Dr Foster Quality Accounts were unable to submit data on compliance with risk assessment, whilst others reported implementation between 0 per cent and 50 per cent."
