Urinary Iodine Quantification in Epidemiological Studies: Optimization in Sample Preparation of the Sandell–Kolthoff Method

We present an optimized Sandell–Kolthoff (SK) spectrophotometric method for urinary iodine quantification, developed to enhance analytical accuracy, robustness, and suitability for high-throughput applications while minimizing reagent consumption. A stepwise refinement of an established protocol (Mod 1–4) was performed by adjusting reagent volumes, digestion temperature, and reaction conditions. The final optimized method (Mod 4) demonstrated excellent calibration linearity (r2 = 0.99964) with a limit of quantification of 10.32 μg/L. Recovery rates for quality controls and certified reference materials ranged from 90 to 105%, with total imprecision (CVtotal) < 3% and analytical bias < 5%. Method comparison with inductively coupled plasma mass spectrometry (ICP-MS) and the Robert Koch Institute (RKI) reference method using Passing–Bablok regression and Bland–Altman analysis confirmed high concordance. Stability testing indicated that iodine concentrations remained unchanged during short-term storage at 4 °C, whereas repeated freeze–thaw cycles caused an average 16% decrease in measured concentrations. Surface contact with urine dipsticks introduced substantial exogenous iodine contamination, resulting in measurement overestimations of up to 45-fold. The optimized SK method provides a validated, precise, and resource-efficient approach for large-scale urinary iodine monitoring, aligning with World Health Organization (WHO) recommendations and addressing key limitations of existing colorimetric protocols.