The risk of blood clots following surgery is probably higher - overall, more than one clot per 50 surgeries - than previously thought, data from the UK’s Million Women Study suggest.

Blood clots can form when patients cannot move for significant periods of time, and for other reasons. Such clots can grow and travel to the lungs, where they can become life-threatening. Doctors can prescribe medications and compression stockings, among other measures, to prevent them.

Earlier studies have suggested that surgery ups the risk as much as 22 times. But in the current study, the risk of such blood clots in the first 6 weeks after inpatient surgery was nearly 70 times higher than it was for those who did not undergo surgery.

And the risk remained higher than average for at least 12 weeks after surgery, study co-author Dr. Sian Sweetland, from the University of Oxford, UK, and colleagues note in the December 4th Online First issue of the journal the BMJ.

Excluding women who had already had a blood clot, as well as those with a history of cancer - which can increase the clotting risk - left almost 950,000 middle-aged subjects in the study. About 240,000 had surgery during the six years in which they were followed, and about 5,400 were admitted to the hospital for blood clots.

Another 270 women died of such clots, autopsies confirmed.

Those who had inpatient surgeries - about 90,000 women - were much more likely to have clots than those who had outpatient surgeries. In the first 6 weeks following surgery, hip and knee replacements had the highest risk, while gynecology procedures had the lowest.

The new findings are “a wake-up call to all surgeons,” Dr. Alexander T. Cohen, from King’s College Hospital, London, writes in a related editorial. In particular, he notes, doctors should consider taking preventive measures for longer than they typically do, for more patients.

SOURCE: BMJ, online December 4, 2009.