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Patient selection

Study defines who needs bariatric surgery most

Raj Padwal (left) and Arya Sharma
Diabetes, gender and smoking status better indicators of risk of death among obese patients

Most patients who undergo bariatric surgery are obese women, even though their male counterparts are more at risk, especially if those men are smokers and have diabetes, according to a study published in JAMA.

Not only do the findings confirm that BMI is not the best way to prioritise patients for bariatric surgery, they outline who should have greater access to surgery.

“If you’re a female non-smoker without diabetes, which, incidentally, is who is being operated on in general around the world, you have the lowest risk,” said principal investigator of the study, Raj Padwal from the Faculty of Medicine & Dentistry, University of Alberta. “Bariatric surgery is most often offered to younger, female non-smokers who don’t have diabetes. “It’s simple math that will predict a patient’s risk of death. For example, if you’re a middle-aged, male smoker with diabetes, your risk of dying in ten years is ten times higher than a young, female, non-smoker who doesn’t have diabetes, irrespective of BMI.”

The researchers created a simple mortality risk calculator that physicians can use to determine the risk of death in patients eligible for bariatric surgery, by inputting age, gender, smoking status and whether the patient has diabetes.


Using current eligibility criteria for bariatric surgery such as BMI thresholds has been criticised as arbitrary and lacking evidence. The study was designed to verify the importance of BMI as a mortality predictor, as well as identify other important mortality predictors, and to construct a mortality prediction rule in a population eligible for bariatric surgery.

They studied individuals from the UK General Practice Research Database, a population-representative primary care registry population-representative register who met contemporary eligibility criteria for bariatric surgery (BMI, ≥35.0 alone or 30.0-34.9 with an obesity-related comorbidity) from January 1988 to December 1998.

They included 15,394 patients and used binary logistic regression to construct a parsimonious model and a clinical prediction rule for ten-year all-cause mortality.


They found that the mean (SD) age was 46.9 (11.9) years, BMI was 36.2 (5.5), and 63.2% of the patients were women. All-cause mortality was 2.1%, and mean follow-up duration was 9.9 years. The final model, which included age (odds ratio, 1.09 per year [95% CI, 1.07-1.10]), type 2 diabetes mellitus (2.25 [1.76-2.87]), current smoking (1.62 [1.28-2.06]), and male sex (1.50 [1.20-1.87]), had a C statistic of 0.768.

Although BMI significantly predicted mortality (odds ratio, 1.03 per unit [95% CI, 1.01-1.05]), it did not improve model discrimination or calibration.

They subsequently divided clinical prediction rule scoring into four tiers. All-cause mortality was 0.2% in tier 1, 0.9% in tier 2, 2.0% in tier 3, and 5.2% in tier 4.

“If we have to decide who should get the surgery first, it should be based on who has the highest risk of mortality,” said Arya Sharma, chair in obesity research and management, scientific director of the Canadian Obesity Network. “We looked at thousands of patient files and many different parameters, and surprisingly enough it came down to three things, if you’re male, you’re a smoker and you have diabetes, you have the highest risk. These surgeries are being done, but are the wrong people getting them? The current BMI cut-off is missing the boat on those who need it most. Having diabetes is more important than BMI as a risk factor. Our research showed BMI didn’t really matter, so size alone isn’t a good way to decide who should get the surgery.”

The research showed that diabetes was the strongest predictor of death, noting that obese patients with diabetes were more than twice as likely to die as obese patients without diabetes. Smoking increased risk of death 1.6 times, and being male increased risk 1.5 times. 

“We think this will be a useful tool for physicians,” added Padwa. “It’s simple math that will predict a patient’s risk of death. For example, if you’re a middle-aged, male smoker with diabetes, your risk of dying in 10 years is 10 times higher than that of a young, female non-smoker who doesn’t have diabetes—irrespective of BMI.”

The research was funded by the Canadian Institutes of Health Research.

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