Iodine status assessment in children: spot urine iodine concentration reasonably reflects true twenty-four-hour iodine excretion only when scaled to creatinine.

Abstract

Currently, the measurement of urinary iodine concentration (UIC, μg/L) is the recommended parameter to assess iodine status, however, the dependency of UIC on urine volume may limit its use as an accurate parameter for monitoring iodine status in populations. Therefore, our objective was to compare two approaches for the assessment of urinary iodine excretion in spot urine samples: UIC (μg/L) and a creatinine-scaled estimate of 24-hour iodine excretion (est24h-UIEcrea [μg/d]) against actually measured 24-hour urinary iodine excretion rates (24h-UIE, μg/d). Urinary iodine and creatinine were measured both in 24-hour urine samples and parallel collected spot urine samples from 180 healthy participants of the DONALD Study, aged 6-18 years. 24h-UIE was used as quasi-reference for actual iodine status. Published 24-hour creatinine reference values served to calculate est24h-UIEcrea. Correlation analysis, cross-classifications, and Bland-Altman plots were used to evaluate agreement between the different assessment approaches. Correlation coefficients of 24h-UIE with UIC (r=0.12, r=0.22; p=n.s.) were substantially weaker than with est24h-UIEcrea (r=0.41, r=0.47; p<0.001) in the 6-12 year old and 13-18 year old groups, respectively. Cross-classification into opposite quartiles by UIC was 7% (6-12 year old group) and 15% (13-18 year old group) versus 5% and 3% by est24h-UIEcrea, respectively. Bland-Altman plots indicated greater deviation from 24h-UIE for the UIC versus the est24h-UIEcrea approach. Our findings in children and adolescents clearly show a better comparability of real 24h-UIE with est24h-UIEcrea than with UIC. Whenever highest possible validity is required for iodine status assessment from spot urine sampling, the determination of est24h-UIEcrea appears to be the more accurate monitoring approach.