Evaluation of automated urinary iodine methods: problems of interfering substances identified.

We evaluated automated methods for measurement of urinary iodine (UI) over a range expected in iodine-replete and iodine-deficient populations. Results obtained with Technicon AutoAnalyzer II systems, based on either dialysis or acid digestion, were compared with those obtained by a manual alkaline ashing technique. Results of automated dialysis were consistently higher than those obtained by the other methods. The apparently higher concentrations of UI we measured were due to interfering substances crossing the dialysis membrane and participating in the catalytic reaction. Thiocyanate (SCN) was one endogenous substance contributing to the increased measurement of UI. For urinary SCN concentrations of 5 to 15 mg/L, the amount of overestimation in the UI measurement attributable to SCN ranged from 21.8 to 61 micrograms/L. However, SCN may account for only 40-50% of the apparent increase in UI. In samples with lower UI (less than 50 micrograms/L), interfering substances produced a 100% error in results. We conclude that the automated dialysis system should not be used to assess iodine-deficient populations. This leaves a major dilemma for researchers wanting to assess the iodine status of populations, because the automated digestion method is no longer commercially available.