Faculty of Medicine & Dentistry researchers Raj Padwal, Sumit Majumdar, Arya Sharma and colleagues published the results of their study in the peer-reviewed journal, JAMA Surgery. Their findings confirm what many have long suspected -- that body mass index (a calculation based on height and weight) is not the best way to prioritize patients for bariatric surgery.

Padwal, Majumdar and Sharma wanted to find out if simple parameters could be used to predict risk of death in obese patients. After reviewing more than 15,000 bariatric surgery patient records from the UK they discovered that sex, smoking status and diabetes accurately predicted those most at risk.

"If we have to decide who should get the surgery first, it should be based on who has the highest risk of mortality," said Sharma. "We looked at thousands of patient files and many different parameters, and surprisingly enough it came down to three things -- if you're a male, smoker and have diabetes, you have the highest risk.

"If you're a female, non-smoker without diabetes, you have the lowest risk, which incidentally is who is being operated on in general around the world. Bariatric surgery is most often offered to younger, female, non-smokers who don't have diabetes.

"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."

Padwal, the principal investigator, said diabetes was the strongest predictor of death, noting obese patients with diabetes were more than twice as likely to die compared to obese patients without diabetes. Smoking increased risk of death 1.6 times, while being male increased risk 1.5 times. Worldwide, about 350,000 bariatric surgeries are performed each year.

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

"We think this will be a useful tool for physicians," Padwal said. "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 a young, female, non-smoker who doesn't have diabetes -- irrespective of BMI."

Sharma, Padwal and Majumdar hope their research will result in changes to clinical practice.