Comparison of the modified S–K method and ICP-MS for urinary iodine measurement: Consistency, accuracy, and vitamin C interference

Iodine status measurement is significant for public health, particularly in pregnant women and individuals with thyroid disease. The current study was to identify the concordance between the modified Sandell-Kolthoff (S-K) assay and inductively coupled plasma mass spectrometry (ICP-MS) assay in measuring urinary iodine concentration, and to evaluate the interference of vitamin C (Vc) level on the measurement accuracy. One hundred fifty-five urine samples were analyzed using both the modified S-K method and ICP-MS with 3 replicate measurements in this prospective study. The samples were categorized into the low Vc level group (qualitative finding of - or ±), the medium Vc level group (qualitative finding of + to 2+), and the high Vc level group (qualitative finding of 3+). Correlation analysis was performed to study the influence of various Vc levels on measuring consistency between the 2 techniques. The modified S-K method showed good overall concordance with ICP-MS (R = 0.458, P < .01), but the mean urinary iodine concentration measured by the modified S-K method was 38.189 μg/L higher than that of ICP-MS (P < .05). Correlation analysis in the different Vc levels showed that with the increasing concentration of Vc, the correlation decreased (R = 0.677, 0.655, and 0.494 for low, medium, and high Vc level groups, respectively). Bland-Altman analysis revealed good agreement, but high Vc level concentrations caused positive bias in the modified S-K method. The S-K method altered, though susceptible to interference, particularly from high Vc level, continues to possess superior correlation with ICP-MS.