The test often misses children with normal BMI who nevertheless carry unhealthy amounts of body fat, researchers from the Mayo Clinic in Minnesota found.
They warned that parents of these children could be given a false sense of reassurance that their child is a healthy weight.
The finding casts doubt on the ability of the UK's National Child Measurement Programme, which measures children's height and weight to determine BMI, to identify correctly all those with weight issues.
Researchers analysed 37 studies of 53,521 patients aged 4 to 18, and compared the effectiveness of BMI to identify excess body fat with techniques such as skin-fold thickness measurements and dual-energy X-ray absorptiometry.
The meta-analysis found BMI has a high specificity in identifying paediatric obesity, meaning it rarely labels healthy children as obese. But it has only moderate sensitivity of 73%, which means it fails to spot around 27% of children who are obese.
Concern over missed cases
Study author Asma Javed from the Mayo Clinic Children’s Center said: 'Our research raises the concern that we very well may be missing a large group of children who potentially could be at risk for these diseases as they get older. We hope our results shine a light on this issue for physicians, parents, public health officials and policymakers.'
The researchers had previously identified a state they called 'normal weight obesity', where adults have a normal BMI but a large percentage of body fat. This state predisposes a person to similar health risks as 'regular' obesity, including pre-diabetes and cardiovascular illness.
Francisco Lopez-Jimenez, an author of the study, said: 'The lesson is that we need additional research in children to determine the potential impact of having high fat in the setting of normal BMI to recognise this issue and perhaps justify the use of body composition techniques to detect obesity at an early stage.'