To evaluate the validity of urinary iodine concentration (UIC) and iodine/creatinine adjustments from spot urines in estimating 24-h urinary iodine excretion (24-h UIE) in school-age children.
A cross-sectional survey in Ningjin and Lingxian Counties, Shandong province, China. Twice repeated 24-h urine and morning spot urine samples were collected from 981 healthy children aged 7-13 years. 24-h UIC, 24-h urinary creatinine concentration (24-h U-Cr), 24-h urinary volume (24-h U-vol), morning spot urinary iodine concentration (MUIC) and morning spot urinary creatinine concentration (MU-Cr) were measured in all samples. Measured 24-h UIE was calculated by 24-h UIC multiplying 24-h U-vol, while estimated 24-h UIE was calculated by morning spot urinary iodine/creatinine ratio (MUI/Cr) multiplying 24-h urinary creatinine excretion (24-h UCrE).
No significant differences were observed in the morning spot urine iodine/creatinine ratio (MUI/Cr), 24-h UIC and measured 24-h UIE between twice repeated collections. The MUI/Cr was significantly lower than 24-h urine iodine/creatinine ratio (24-h UI/Cr) in both collections, and the estimated 24-h UIE was lower than measured 24-h UIE as well. Measured 24-h UIE and estimated 24-h UIE were significantly correlated with age. However, no differences were observed in 24-h UIC and MUIC among different ages. In addition, MUI/Cr, 24-h UI/Cr and estimated 24-h UIE were strongly correlated with measured 24-h UIE.
Iodine/creatinine adjustments from spot urine could be a valid and reliable alternative to the measured 24-h UIE in evaluating individual iodine intake, but UI/Cr adjustments from morning spot urine may underestimate urinary iodine excretion in school-age children.