[Urinary iodide analysis: critical study of the digestion method]

Urinary iodine is largely measured in microtiter plates by a colorimetic ceric-arsenic assay based on the Sandell-Kolthoff reaction. However, a preliminary digestion step is necessary and requires a particular care not only to transform all the iodo-compounds into iodide but also to prevent the formation of substances liable to the disturb of the subsequent redox reaction. In the present study we tested three types of digestion processes, among them two conventional methods (ammonium persulfate and chloric acid) and a new one using combined nitric acid/hydrochloric acid. Results showed that important errors may be obtained with the chloric acid and the ammonium persulfate digestions. These discordances were the consequence of either an incomplete transformation of iodo-compounds or an oxidation of iodide into molecular iodine or a colorimetric assay disturbance due to a residual yellow coloring. No problems were evidenced with the combined nitric acid/hydrochloric acid process, which remains the better alternative to evaluate the urinary iodine. It could also provide a particularly useful means of assessing the iodine status in epidemiological studies.