Background: Weight for height in children is often assessed by comparing the child's weight-for-age centile with their height-for-age centile. However, this assessment has not been validated statistically, and it differs from the body mass index (BMI) centile.
Objective: To study indices of weight-for-height based on weight centile-for-age adjusted for height centile-for-age, and to see how they relate to the BMI centile-for-age.
Design: Cross-sectional survey of data for 40 536 boys and girls aged 0-18 y from the 1980 Nationwide Dutch Growth Survey, using the British 1990 and US CDC 2000 growth references.
Outcome measures: Two measures of weight for height: (a) the difference between weight centile and height centile, and (b) BMI centile, with the centiles analysed as SD scores (SDS).
Results: BMI centile is correlated strongly with weight centile (r=0.77) but weakly with height centile (r=0.1). By contrast the difference between weight centile and height centile is correlated only weakly with weight centile (r=0.3) and strongly negatively with height centile (r=-0.5). BMI centile is predicted to high accuracy by the multiple regression on weight centile and height centile (93-97% of variance explained, s.e.e. 0.2 units).
Conclusions: A child's BMI centile can be calculated to high accuracy from their weight and height centiles as read off the weight and height charts. This avoids the need to calculate BMI or to plot it on the BMI chart. A chart is provided to simplify this calculation, which works throughout the spectrum of nutritional status. It can also be used to monitor individuals' weight, height and BMI centiles simultaneously as they change over time. However the simpler procedure of comparing weight and height centiles (eg a difference of two or three channel widths) is a poor measure of weight-for-height and should not be used.