Alternative to BMI for diagnosing obesity could prevent misdiagnosis in 'tens of millions'

Medical experts have long considered body mass index a blunt instrument for defining obesity, leading to concerns substantial numbers of people with excess body fat are missed, while others who are healthy are wrongly diagnosed.

An alternative to BMI for diagnosing obesity could prevent misdiagnosis in millions of people worldwide, according to doctors who found it more accurately distinguished healthy people from those with harmful excess body fat.

Medical experts have long considered body mass index a blunt instrument for defining obesity, leading to concerns substantial numbers of people with excess body fat are missed, while others who are healthy are wrongly diagnosed.

The new approach, which combines BMI with other body measurements and organ function tests, suggests BMI alone misses obesity in 5-9% of adults, while wrongly diagnosing the condition in as many as 3%.

“This misclassification can account for tens of millions of people, so the inaccuracy of BMI is a big problem at scale and also for individuals,” said Francisco Rubino, a professor of metabolic and bariatric surgery at King’s College London.

BMI is calculated by dividing weight in kilograms by height in metres squared. About 1.6 billion people worldwide are classified as overweight, meaning their BMI falls between 25 and 29.9, and nearly a billion with a BMI of 30 or more are considered obese. 

More than 70% of adults in the US are overweight or obese based on BMI alone.

Rubino and his colleagues drew on health records from more than 800,000 people in Britain, US, China and Spain to test a diagnostic approach for obesity that he and other world experts proposed last year in a Lancet Commission report. 

They used the records to track people’s health for up to 15 years.

Under the new definition, someone with a BMI over 40 is considered obese, but doctors check people with a lower BMI for excess body fat by looking at their waist measurement.

A BMI of 30-plus with a raised waist measurement or waist-to-height ratio counts as obese, as does a BMI from 25 to 29.9 if waist measurement and waist-to-height ratio are high. 

In most countries, a raised waist measurement means more than 102cm (40in) for men and 88cm for women, while thresholds for a high waist-to-height ratio are about 0.57.

In a second step of the diagnosis, doctors distinguish between clinical obesity, meaning an active disease where excess body fat is already impairing organ function, and pre-clinical obesity, where the extra fat is a health risk but has yet to cause problems.

Obesity levels rose only slightly overall under the new definition, and only a quarter to a third of obese people, or 5% to 10% of the wider population, who had clinical obesity and a substantially higher risk of death.

The approach allows doctors to distinguish between healthy and sick people who have the same BMI. Three people with a BMI of 32, for example, would all be classed as obese but would have very different diagnoses. 

One might have no excess body fat and be healthy, another could have excess fat but no organ dysfunction and have pre-clinical obesity, and the third could have excess fat and organ dysfunction and be clinically obese.

Writing in Nature, the scientists describe how some people with a high BMI were not obese under the new definition and actually had a lower mortality risk than others considered healthy. Reassuringly, 80% to 90% of people with pre-clinical obesity did not progress to clinical obesity, or active disease, in the next 10 years.

The approach makes the process more complex for clinicians, but Rubino said people should be diagnosed for obesity at the individual level to ensure the best healthcare, whether that involved lifestyle advice, GLP-1 weight-loss drugs or surgery.

The Guardian