The impact of school-day variation in weight and height on National Child Measurement Programme body mass index-determined weight category in Year 6 children

Background In England, the National Child Measurement Programme (NCMP) annually measures the weight and height of Year 6 schoolchildren (age 10-11 years). While measurement protocols are defined, the time of measurement within the school day is not. This study examined the impact of school-day variation in weight and height on NCMP body mass index (BMI)-determined weight category in Year 6 children. Methods Standing height and weight were measured in morning and afternoon sessions in 74 children, boys (n = 34; height: 141.16 +/- 7.45 cm; weight: 36.48 +/- 9.46 kg, BMI: 18.19 +/- 3.98 kg/m(2)) and girls (n = 40; height: 144.58 +/- 7.66 cm; weight: 42.25 +/- 11.29 kg; BMI: 19.97 +/- 3.98 kg/m2) aged 11 +/- 0.3 years. Results In the whole sample, height decreased (Mean = -0.51 cm, 95% CI: -0.39 to -0.64 cm, P = 0.01), weight did not change (Mdn = 36.40 to 36.35, P = 0.09) and BMI increased (Mdn = 18.04 to 18.13, P = 0.01). In girls weight increased (Mdn = 41.40 to 41.60, P = 0.01). BMI percentile increased (Mdn = 57th to 59.5th centile, P = 0.01). One girl increased in BMI category from morning to afternoon according to the clinical cut-offs (<= 2nd, >91st and >98th) and three girls increased BMI category according to the population monitoring cut-offs (<= 2nd, >= 85th, >= 95th). Conclusions School-day variation in height (and in girls alone, weight) impact upon increased BMI and BMI percentile in afternoon versus morning measurements in Year 6 children. Although not reaching statistical significance, resultant variation in categorization at the individual level may lead to unwarranted follow-up procedures being initiated. Further research with larger samples is required to further explore the impact of daily variability in height and weight upon both clinical and population monitoring BMI-determined weight status categorization in the NCMP.