Measuring and interpreting BMI in Children

It is important when using BMI in children that age and sex appropriate growth references are used to correctly determine weight status. In England the British 1990 (UK90) growth reference charts should be used, to determine the weight status of an individual child and population of children.

A review of the: Consideration of issues around the use of BMI centile thresholds for defining underweight, overweight and obesity in children aged 2-18 years in the UK, was published by the Scientific Advisory Committee on Nutrition and the Royal College of Paediatrics and Child Health in April 2012.

Measuring an individual child:

Clinical definitions of weight status: When measuring an individual child (for example in clinic or feeding back NCMP results to parents) weight status is defined using the UK90 growth charts clinical cut points which are as follows:

Clinically very underweight: ≤ -8/3 BMI z score (≤0.4th rounded centile)
Clinically low weight: ≤ -6/3 BMI z score (≤2nd rounded centile)
Clinically Healthy weight: > -6/3 to <4/3 BMI z score (>2 to <91st rounded centile)
Clinically Overweight: ≥4/3 BMI z score (≥ 91st rounded centile)
Clinically Obese*: ≥ 6/3 BMI z score (≥98th rounded centile)
Clinically Extremely obese: ≥ 8/3 BMI z score (≥99.6th rounded centile)

*This is also called ‘very overweight’ in the NCMP parental feedback letters.

The lines on the UK90 growth charts show a 2/3rds BMI z score or SD (Standard Deviation) spacing: -8/3, -6/3, -4/3, -2/3, 0, 2/3, 4/3, 6/3, 8/3, which round to the 0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th, and 99.6th centiles.

Calculating BMI centiles for individual children:

The BMI centile of individual children can be calculated using the NHS choices BMI calculator: http://www.nhs.uk/Tools/Pages/Healthyweightcalculator.aspx

It is important to visit a health care professional if any parent or child is concerned about their weight.

Measuring a group or population of children:

Population monitoring definitions of weight status:

When measuring a population of children (for example reporting NCMP findings) weight status is defined using the following UK90 population cut points. These cut points are slightly lower than the clinical cut points, this is to capture those children with a weight problem and those at risk of developing a weight problem (ie those children who maybe on the border line of the clinical definition). This helps ensure that adequate services are planned and delivered for the whole population.

Underweight: ≤2nd centile
Healthy weight: >2 - <85th centile
Overweight: ≥ 85th centile
Obese: ≥95th centile

Calculating BMI centiles for child populations:

In order to calculate BMI centiles for large numbers of children, the simplest and most accurate method is to use the ‘LMS Growth’ Microsoft Excel add-in software. Published at:

http://www.healthforallchildren.com/?product=lmsgrowth

This software can also be used to calculate centiles and z/SD scores for BMI and other child measurements such as height, weight, and waist circumference, and is available at no charge from Harlow

NB when using BMI z or SD (Standard Deviation) scores, the 99.6th centile equates to 8/3 SD (2.66667), not the +3 SD (as often presumed). This is the ‘top’ threshold shown on the BMI lookup within the UK90 growth charts, and the 99.6th is the top centile on the height and weight charts. The reason for this is that the growth charts show a 2/3rds SD spacing: -8/3, -6/3, -4/3, -2/3, 0, 2/3, 4/3, 6/3, 8/3, which round to the 0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th, and 99.6th centiles.