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/m²) 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.