BMI tests 'miss' over a quarter of obese children

"Quarter of obese children missed by BMI tests," the Mail Online reports.

The headline was prompted by a review that combined the results of 37 studies in more than 50,000 children and found body mass index (BMI) is an imperfect way of detecting excess body fat.

The study estimated more than a quarter of children (27%) with excess body fat might not be classified as obese when using BMI measurements alone.

This may mean the missed children don't get the same support to achieve a healthy weight as those correctly identified as obese, and so remain at a higher risk of developing a range of weight-related diseases, such as type 2 diabetes.

BMI has long been known to be a relatively blunt tool in terms of accurately assessing body fat, as demonstrated in a similar study in 2012. However, this latest study puts a specific figure on the imperfection.

That said, BMI remains a very useful tool. It provides a reasonably accurate method to estimate obesity rates at a population level, taking just minutes to complete.

Other methods can be more resource and time consuming (hydrostatic weighing), or can have large margins of error if not done correctly (skin callipers).

Overall, this study adds to the evidence of BMI's "bluntness" by quantifying the possible impact of the inaccuracy.

If you are worried about your child's weight, contact your GP. They should be able to make a more detailed assessment.

Where did the story come from?

The study was carried out by researchers from the US and Czech Republic. No funding source was reported.

It was published in the peer-reviewed medical journal, Pediatric Obesity.

The Mail Online's coverage was broadly accurate, though if we were being really pedantic we would point out to the headline writers that 27% is not the same as 25%.

What kind of research was this?

This was a systematic review and meta-analysis of studies assessing the diagnostic performance of BMI to detect excess fat in children up to the age of 18.

A systematic review seeks to identify and pool the results of all published material on a specific topic, and is an effective way of summarising lots of research evidence. A meta-analysis is a related statistical exercise, where the results of studies are pooled. 

Excess fat in people raises the risk of many weight-related diseases, such as diabetes and heart disease. Detecting excess fat in children helps identify those most at risk of damaging their health.

The researchers point out the ideal way of identifying obesity in children and adolescents has not been determined, although BMI is the most widely used screening tool.

This involves weighing and measuring the height of a young person to estimate their BMI. The BMI is then compared against standard cut-offs, which categorises the person as either underweight, a healthy weight, overweight or obese.

In England, this is the approach adopted by the NHS National Child Measurement Programme.

What did the research involve?

The researchers searched electronic medical databases for studies assessing the performance of BMI measurement compared with other measures of body fat in people less than 18 years of age.

They then pooled the individual study findings using a meta-analysis to give an overall estimate of how well BMI identified people with excess body fat.

All of the studies included had to compare measuring body fat using BMI with a different reference method, such as DEXA.

Study authors of relevant articles published on the topic were contacted to source additional relevant literature and supplement the electronic database searches.

The main analysis reported the sensitivity and specificity of using BMI to detect excess fat in males and females.

The analysis explored variation between the studies with regard to differences as a result of race, BMI cut-off, BMI reference criteria, and the reference standard for assessing fatness.

What were the basic results?

The analysis included 37 studies involving 53,521 patients. The average age in the studies ranged from 4 to 18 years.

The main finding was that commonly used BMI cut-offs showed a pooled sensitivity to detect high fatness of 0.73 (95% confidence interval [CI] 0.67 to 0.79) and a specificity of 0.93 (95% CI 0.88 to 0.96).

This means that BMI correctly identified children with high fat levels 73% of the time, and correctly identified children without high fat levels 93% of the time.

On the flip side, this means up to 27% (100% minus 73%) of children with high fat levels were not correctly identified using BMI, so 27% was the false positive rate.

There was moderate variation in the pooled results as a result of the confounders mentioned above.

How did the researchers interpret the results?

These results led the researchers to conclude that, "BMI has high specificity but low sensitivity to detect excess adiposity [body fatness] and fails to identify over a quarter of children with excess body fat percentage."

Conclusion

This systematic review and meta-analysis showed using BMI to detect excess body fat in children up to the age of 18 was not perfect. It estimated more than a quarter of children with excess body fat might not be classed as obese using BMI measurements alone.

This may mean they don't get the same help and support to achieve a healthy weight as those correctly identified, and so remain at a higher risk of developing a range of weight-related diseases.

BMI is known to be a far from perfect measure of body fatness, but is often a useful start, so the main conclusion of the research will be nothing new to many health professionals.

However, this study has put a specific figure on the imperfection: more than 25% are incorrectly given the all-clear when their weight may be harming their health.

England's current screening of children for excess body fat, the National Child Measurement Programme, uses BMI as its main measure, so this is very relevant to England's youth.

The way excess body fatness is assessed in this programme is regularly assessed, and this study may contribute to the evidence base considered at the next review of the methodology.

Measuring body fat in children on a large scale is a challenge, and the best way to do this is likely to be balancing accuracy with pragmatism. Some measures of body fat are time consuming to perform and, in the context of a busy school environment, this may be influential.

Overall, this study raises a known issue with using BMI to assess body fat in children, but adds to the evidence by quantifying the possible impact of the inaccuracy.

If you have any concerns about your child's weight, your GP will be able to assess whether their weight may be affecting their health and can offer help and support.