Age standardization of weight-for-height in children using a unified Z-score method

Weight-for-height standards in children are usually constructed on the basis that the expected weight for a given height does not depend on age, an assumption which is unjustified. Analysis of data from a Shiraz (Iran) growth study confirms the need to take age into account when assessing weight-for-height of schoolchildren aged 6-12. The method described by Cole in 1979 (Annals of Human Biology, 6, 249-268) leads to errors in the assessment of these children, in particular 6-year-old boys and 10-year-old girls. This paper proposes an alternative, which relies on Z-score of height-for-age (HI) and Z-score of log (weight)-for-age (WI). These indices are age- and sex-specific standard scores (Z-scores) derived from modelling height-for-age and weight-for-age smoothed centile curves. The method provides a unified structure of the regression of WI on HI, which overcomes the problem of age dependency in weight-height relationship. Age-adjusted smoothed centiles of weight-for-height are derived at each age of weight and height to their respective Z-scores, and weight-for-height centiles of 1207 Shiraz schoolchildren in a representative sample in a cross-sectional study are drawn for each age-sex category. This Z-score method proves to be simple and stable, and is likely to be useful for any sort of growth data.