RESUMO
BACKGROUND: Current guidance on the optimum interval between measurements in infancy is not evidence based. We used routine data to explore how measurement error and short-term variation ('noise') might affect interpretation of infant weight and length gain ('signal') over different time intervals. METHOD: Using a database of weights and lengths from 5948 infants aged 0-12 months, all pairs of measurements per child 2, 4 and 8 weeks apart were extracted. Separately, 20 babies aged 2-10 months were weighed on six occasions over 3 days to estimate the SD of the weight difference between adjacent measurements (=116 g). Values of 116 g and 0.5 cm for 'noise' were then used to model its impact on (a) the estimated velocity centile and (b) the chance of seeing no growth during the interval, in individuals. RESULTS: The average gain in weight and length was much larger than the corresponding SD over 8-week and 4-week time intervals, but not over 2 weeks. Noise tended to make apparent velocity less extreme; after age 6 months, a 2-week velocity that appeared to be on to the ninth centile, would truly be on the second-third centile if measured with no noise. For 2-week intervals, there was a 16% risk of no apparent growth by age 10 months. CONCLUSIONS: Growth in infancy is so rapid that the change in measurements 4-8 weeks apart is unlikely ever to be obscured by noise, but after age 6 months, measurements 2 weeks or less apart should be treated with caution when assessing growth faltering.
