Standards for validating health measures definition and content

Figure 1 shows centiles for body mass index by sex based on the British reference,9 with seven centiles spaced two thirds of a z score apart—that is, z=−2, −1.33, −0.67, 0, 0.67, 1.33, and age 18 is z score 1.19 in females, corresponding to the 88th centile, and 1.30 in males, on the 90th centile.Clearly a cut off point related to age is needed to define child obesity, based on the same principle at different ages, for example, using reference centiles.10 In the United States, the 85th and 95th centiles of body mass index for age and sex based on nationally representative survey data have been recommended as cut off points to identify overweight and obesity.11 For wider international use this definition raises two questions: why base it on data from the United States, and why use the 85th or 95th centile?Other countries are unlikely to base a cut off point solely on American data, and the 85th or 95th centile is intrinsically no more valid than the 90th, 91st, 97th, or 98th centile.This process is repeated for all six datasets, by sex.

A body mass index of 30 kg/m at age 18 is on the 99th centile in both sexes, an obesity prevalence of about 1%.Each z score substituted into equation 1 provides the formula for an extra centile curve passing through the specified point (dotted line in fig 1).Each centile curve defines cut off points through childhood that correspond in prevalence of overweight or obesity to that of the adult cut off point—the curve joins up points where the prevalence matches that seen at age 18.which summarises the data in terms of three smooth age specific curves called L (lambda), M (mu), and S (sigma).The M and S curves correspond to the median and coefficient of variation of body mass index at each age whereas the L curve allows for the substantial age dependent skewness in the distribution of body mass index.Regardless of centile or reference population, the cut off point can still be criticised as arbitrary.


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