Overweight and obese middle-aged women have increased risk of venous thromboembolism, particularly following surgery
- Correspondence to
Dr Simon McRae
Department of Haematology, Royal Adelaide Hospital, North Terrace, Adelaide, SA 5156, Australia;
Commentary on 
Implications for practice and research
Obesity is associated with an increased risk of venous thromboembolism (VTE) including both spontaneous and postoperative events.
Further research is needed regarding factors responsible for the increase in postoperative VTE in overweight patients, and whether management should be altered (eg, increased focus on postoperative mobilisation, altered doses of prophylactic anticoagulants, lower threshold for starting prophylactic anticoagulation for minor surgical procedures).
Weight loss prior to surgery may reduce risk of venous thrombosis.
About 0.1% of individuals develop VTE each year.1 Previous case–control studies, suggest that being overweight (body mass index (BMI) >25–29.9 kg/m2) or obese (BMI>30 kg/m2) results in an approximate twofold increase in VTE risk.2 Data are limited regarding the influence of obesity on the risk of postoperative VTE following non-orthopaedic surgery. This large prospective study is aimed at more accurately estimating VTE risk associated with obesity, particularly in the postoperative setting.
This large observational study recruited 1.13 million women between 1996 and 2001 from the NHS breast screening programme. Women with prior VTE, cancer or recent surgery were excluded. Data were collected by questionnaire on height, weight and other risk factors for thrombosis (eg, HRT and physical activity). Participants were followed from the time of study entry until 2008 or the development of cancer or death. Individual hospital admissions were flagged by linkage to English Hospital Episode Statistics and the Scottish Morbidity Records, and discharge coding was used to identify patients who had undergone surgery and/or who had developed VTE. The relative risk (RR) of undergoing surgery and hospital admission or death due to VTE according to BMI was estimated from HR using a Cox regression model adjusted for socioeconomic group, exercise frequency, hormone therapy, smoking, hypertension and diabetes.
A total of 1 170 495 women (average age 56.1 years) participated in the study; of which 46.8% were of normal weight (BMI<25 kg/m2), 35.7% were overweight and 17.5% obese. Of these, 0.4% were admitted to a hospital or died as a result of VTE. Fifty-five per cent of participants underwent surgery, with women who were overweight (RR 1.1) or obese (RR 1.2) being more likely to undergo a surgical procedure. In women not undergoing surgery, the risk of VTE increased with higher BMI. Compared with women with a BMI of 22.5–24.9, there was an approximate twofold increase in VTE risk with a BMI of 30–35 and a 3.5-fold increase with a BMI>35.0. The risk of VTE in the 12 weeks following surgery increased 1.5-fold in overweight women and 1.8-fold in obese women.
This is the largest study till date examining obesity as a risk factor for VTE, with twice the number of VTE events included than that in prior studies.2 Study strengths include a large sample size, exclusion of only a small proportion of individuals from the initial cohort (∼5%), almost complete follow-up data regarding VTE-related hospital admission and death, separation of non-operative and postoperative VTE and appropriate adjustment for known thrombotic risk factors. While BMI was based on self-reported height and weight, this correlated well with measured BMI in a subsample of the cohort. The validity of the study findings is reinforced by the consistency between the estimated increase in risk of non-operative VTE with obesity (2–3-fold) and the results of previous publications.2 ,3
Episodes of VTE treated entirely in outpatient settings (most likely DVT) will not have been identified in this study. The reported 12-week VTE incidence rates, ranging from 0.1/1000 women with BMI<25 not undergoing surgery to 7.0/1000 women with BMI>25 after major surgery, are therefore likely to be underestimated, however, the relative risk is likely to remain accurate. As the study is restricted to women, research is required to confirm the findings in men.
The increase in the risk of postoperative VTE in overweight patients after a broad range of surgical procedures highlights the importance of preventative measures in this population. The study does not clarify whether an increase in VTE risk with raised BMI is more pronounced with certain surgical types. Research is needed to understand the cause of the increase in VTE risk, that is, associated hypercoagulability, decreased mobility and ineffectiveness of prophylactic anticoagulation,4 that may allow targeted measures to diminish thrombosis risk.