Intra-individual variation in urinary iodine concentration: effect of statistical correction on population distribution using seasonal three-consecutive-day spot urine in children.

OBJECTIVE To determine the effect of statistical correction for intra-individual variation on estimated urinary iodine concentration (UIC) by sampling on 3 consecutive days in four seasons in children. SETTING School-aged children from urban and rural primary schools in Harbin, Heilongjiang, China. PARTICIPANTS 748 and 640 children aged 8-11 years were recruited from urban and rural schools, respectively, in Harbin. PRIMARY AND SECONDARY OUTCOME MEASURES The spot urine samples were collected once a day for 3 consecutive days in each season over 1 year. The UIC of the first day was corrected by two statistical correction methods: the average correction method (average of days 1, 2; average of days 1, 2 and 3) and the variance correction method (UIC of day 1 corrected by two replicates and by three replicates). The variance correction method determined the SD between subjects (Sb) and within subjects (Sw), and calculated the correction coefficient (Fi), Fi=Sb/(Sb+Sw/di), where di was the number of observations. The UIC of day 1 was then corrected using the following equation:[Formula: see text] RESULTS The variance correction methods showed the overall Fi was 0.742 for 2 days' correction and 0.829 for 3 days' correction; the values for the seasons spring, summer, autumn and winter were 0.730, 0.684, 0.706 and 0.703 for 2 days' correction and 0.809, 0.742, 0.796 and 0.804 for 3 days' correction, respectively. After removal of the individual effect, the correlation coefficient between consecutive days was 0.224, and between non-consecutive days 0.050. CONCLUSIONS The variance correction method is effective for correcting intra-individual variation in estimated UIC following sampling on 3 consecutive days in four seasons in children. The method varies little between ages, sexes and urban or rural setting, but does vary between seasons.