24-Hour Urine Samples Are More Reproducible Than Spot Urine Samples for Evaluation of Iodine Status in School-Age Children.

BACKGROUND Variation in different urinary measurements for evaluation of iodine status is of concern to clinicians and researchers. OBJECTIVE This study evaluated variations between urine iodine concentration (UIC), spot and 24-h urine sample creatinine concentrations, and 24-h urine iodine excretion (24-h UIE) in repeated samples from school-age children. METHODS Urine samples (24 h and morning spot) were collected on 2 occasions from 981 children in Ningjin and Lingxian counties, China. Samples from Ningjin were collected in October and November 2013, and samples from Lingxian were collected in April and May 2014. Morning spot urine iodine concentration (MUIC), morning spot urine creatinine, 24-h UIC, and 24-h urine creatinine were measured in all samples. The 24-h UIE was calculated by multiplying the 24-h UIC by the 24-h urine volume. RESULTS In Ningjin County, the 24-h UIC and 24-h UIE did not differ between repeated collections [192 and 172 μg/L, respectively, for 24-h UIC (P = 0.08); 123 and 120 μg/L, respectively, for 24-h UIE (P = 0.56)], whereas the MUIC was lower in November 2013 than in October 2013 (170 and 190 μg/L, respectively; P = 0.034). In Lingxian County, no significant differences were observed in 24-h UIC between the repeated collections (230 and 218 μg/L, respectively; P = 0.79), whereas the 24-h UIE and MUIC were higher in the samples collected in May 2014 than in April 2014 [161 and 155 μg/L, respectively, for 24-h UIE (P = 0.002); 244 and 203 μg/L, respectively, for MUIC (P < 0.001)]. When data from both counties were combined, no difference was observed between repeated 24-h UIC (214 compared with 196 μg/L; P = 0.17) and 24-h UIE (143 compared with 143 μg/d; P = 0.06), but MUICs were lower in the first collection than in the second collection (199 and 207 μg/L, respectively; P = 0.002). The κ values were >0.4 for 24-h UIC and mean UIE, whereas relatively low κ values were observed for MUIC and mean UIE. CONCLUSION The 24-h UIC was more accurate and reproducible than the MUIC in evaluating iodine status in a large-scale population study of school-age children.