Friday December 4 2009
Scan showing a deep vein thrombosis in the left calf
“Patients recovering from surgery are at a high risk of fatal blood clots for much longer than thought,” the Daily Mail reported. It said researchers had found that the risk continues for up to three months after surgery.
This report is based on a large study in 947,454 middle aged women, which examined the risks of developing blood clots after having different types of surgery. It found that there was an increased risk of blood clots up to 12 weeks after surgery.
This research has some uncertainties but due to its size these findings appear to be reliable. While it was already known that the risks were greatest in the few weeks following surgery, this study suggests that the risk may continue for a substantial time beyond this period. These findings may have implications for the use of blood clot treatments after surgery.
Where did the story come from?
The research was carried out by Dr Siân Sweetland and colleagues from The Cancer Epidemiology Unit at the University of Oxford. The study was funded by Cancer Research UK and the Medical Research Council. It was published in the peer-reviewed British Medical Journal.
Generally, the story was reported similarly well and accurately by The Daily Telegraph, The Daily Mail and BBC News.
What kind of research was this?
This was a large prospective cohort study involving 947,454 women who were followed for an average of 6.2 years. The research aimed to examine the risk of venous thromboembolism (blood clot) after different types of surgery. The researchers say that the risk of venous thromboembolism is highest during the first few weeks after surgery, but the exact pattern and magnitude of this over time is uncertain.
As the incidence of blood clots is relatively low in the general population, such a large study is necessary to give representative information.
What did the research involve?
The participants were part of the Million Women Study. This is a population-based prospective study that recruited 1.3 million women through the NHS breast screening programme between 1996 and 2001. The average age of the participants was 56 years and the majority were postmenopausal.
These women were followed up until 2005, for an average of 6.2 years. Their inpatient and day-case hospital admission records were analysed for this period. The researchers compared two groups of women:
- Participants who had undergone day case or inpatient surgery during the follow-up period.
- Participants who had not had any surgery over that period.
The researchers then checked hospital records for the incidence of blood clots. They grouped hospital admissions for two types of clots: deep vein thrombosis and pulmonary embolisms (clots affecting blood vessels in the lungs). They also looked at the incidence of blood clots with different types of surgery and the likelihood of having a blood clot with increasing time after surgery.
In their analysis, the researchers also took into account other factors that can affect the likelihood of having a blood clot, such as a high body mass index or being on hormone replacement therapy (HRT).
What were the basic results?
The women in the two groups were of similar age, weight, postmenopausal status and lifestyle, when they first enrolled in the study.
Both inpatient surgery and day-case surgery increased the risk of blood clots occurring within six weeks post operation, compared with people who had not had surgery. Women who had day-case surgery were 10 times more likely to have blood clots than women who had no surgery, (relative risk 9.6 [95% confidence interval, 8.0 to 11.5]) The risk for women who had inpatient surgery was almost 70 times higher (relative risk 69.1 [95% confidence interval, 63.1 to 75.6]).
In the inpatient group, the risk of blood clots peaked three weeks after surgery. The risk of clots in the day case group steadily decreased from immediately after the surgery. This risk reduced over time, but there was a slight increase in risk that was statistically significant 12 months after surgery.
At seven to 12 weeks after surgery, the risk of blood clots was six times higher for the day case surgery group compared to the no-surgery group, and 20 times higher for the inpatient surgery group.
Different types of surgery required a different length of stay in hospital afterwards. Patients who had knee or hip surgery, had an average stay of eight days in hospital and were more than 200 times more likely to have blood clots in the six weeks following surgery than a person who had not had surgery. When the researchers looked at the absolute incidence of blood clots in the 12 weeks after surgery they found:
- One in 815 patients developed blood clots after day-case surgery,
- One in 140 patients developed blood clots after inpatient surgery. This increased to 1 in 45 following knee or hip surgery.
- For women who had not had any surgery, about one in 6,200 developed clots for the same period.
How did the researchers interpret the results?
The researchers conclude that in the year following an operation, the risk for hospital admission varies considerably. “Incidence rates for venous thromboembolism (blood clots) in the first six postoperative weeks were over 100 times the rates without surgery and, seven to 12 weeks after inpatient surgery rates, were still almost 20 times higher than without surgery. The relative risks after day-case surgery were lower than for inpatient surgery but still substantially increased.”
The researchers also say that the risks are greater and last longer than previously thought. They suggest that, following an operation, the time that patients are given the drugs to prevent blood clots should be extended to 12 weeks.
In this large and well-conducted study, the researchers determined that the risk for blood clots after surgery could last up to 12 weeks after the procedure. They also calculated the different risks for different types of surgery.
They warn that the incidence of blood clots after surgery may actually be higher than their figures indicate. This is because it’s likely that women were given treatments to prevent blood clots after surgery, and because the reported number based on hospital records may be an underestimation as some types of blood clot do not have symptoms.
It is worth noting that the researchers had no data on how many of the participants were taking preventive measures, such as wearing stockings or taking blood-thinning drugs around the time of their surgery. They say, quite reasonably, that if such data had been taken into account, the increased risk might have been even greater for people who had surgery and were not taking any preventive measures.
However, despite these uncertainties, and due to its size, these findings appear to be reliable. While it was already known that the risks were greatest in the few weeks following surgery, this study suggests that the risk may continue for a substantial time beyond this period. These findings may have implications for the use of blood clot treatments after surgery.