Urine test strips as a source of iodine contamination.

In the course of conducting an analysis in a subset of the controlled antenatal thyroid screening study (1) samples, we recently noted that some urine samples from pregnant women in Wales, a region known to be iodine deficient, had urinary iodine concentrations in excess of 500mg=L. We suspected contamination from urine test strips. We obtained Combur Test D urine test strips (Roche Diagnostics, Mannheim, Germany), as were used in the Welsh study. We also obtained Multistix 10 SG urine test strips (Bayer Health Care, Elkhart, IN), used in our clinic, for comparison. Iodine was not mentioned in the package insets for either test strip. Two urine samples were obtained from healthy volunteers. Total urinary iodine concentrations were measured both spectrophotometrically by a modification of the method of Benotti et al. (2) and by mass spectroscopy. Measurements were obtained at baseline, after placing each test strip in the urine for 1 second, and again after placing each test strip in the urine for 30 seconds (Table 1). To determine which individual tests contained iodine, the test strips were cut into separate pieces for each test, and urine iodine concentrations were remeasured after placing individual pieces in a separate urine sample for 30 seconds. Iodine was present in the tests for glucose and blood in the Combur Test D and only in the Multistix test for glucose. The test strips for glucose rely on sequential reactions. First, glucose oxidase catalyzes the aerobic oxidation of glucose to gluconic acid and hydrogen peroxide. Second, in the presence of iodide, hydrogen peroxide oxidizes the iodide to free iodine, producing a color change in the indicator. The color change produced indicates the amount of hydrogen peroxide present, and therefore the glucose content of the urine. Although previously reported by Chanoine et al. (3) over 20 years ago, the contamination of urine with iodine by test strips is not well known. Since Combur and Multistix test strips still contain iodine as a reagent for blood and glucose testing, urine iodine concentrations should be assessed before test strip measurements. This is important in evaluating populations for iodine sufficiency. References