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

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.