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Effect of high room temperature on urinary iodine assay.

Assay of urinary iodine, along with other measurements, is essential for assessing the iodine status of populations in iodine-deficient areas and for monitoring the effectiveness of control programs. The International Council for Control of Iodine Deficiency Disorders has published a number of urinary iodine assays that are applicable to specific situations [1]. One commonly used method, their "Method A," is based on an initial mild chloric acid digestion to remove interfering substances, after which the iodine is quantified by its catalytic effect on reducing yellow ceric ion to colorless cerous ion in the presence of arsenious acid (Sandell-Kolthoff reaction). This is usually done manually in a colorimeter at 405 nm, the details having been described by Dunn et al. [2]. In brief, arsenious acid solution is added to each digested urine, blanks, and calibrators. After 15 min, ceric ammonium sulfate is added to each tube and all tubes are left at room temperature for exactly 20 min, at which time the remaining absorbance at 405 nm is measured.

In establishing this method in Hanoi, Vietnam, during the summer months, we consistently obtained very low absorbance readings for all tubes, including the blanks, and so made attempts to remove the supposed iodine contamination. The derived calibration curves were too flat to be useful. We then realized that the low absorbance readings, determined with a Jenway Model 6100 monochromator-type spectrophotometer (Felsted-Dunmow, Essex, UK), actually reflected rapid catalytic conversion of ceric to cerous ions-the speed of the reaction probably being due to the high ambient room temperatures (measured at 33-35 [degrees]0). This conjecture was confirmed by performing the catalytic process at various lower temperatures in a water bath cooled with ice (Table 1).

Iodine deficiency occurs in many of the hotter equatorial regions, some of which are served by laboratories that cannot afford efficient air conditioners. Where laboratorians encounter this problem of rapid catalysis of ceric to cerous ions, they may resolve it by using a cooled water bath or by installing air conditioning if funds are available.

References

[1.] Dunn JT, Crutchfield HE, Gutekunst R, Dunn AD. Methods for measuring iodine in urine. The Netherlands: International Council for Control of Iodine Deficiency Disorders, 1993:18-51.

[2.] Dunn JT, Crutchfield HE, Gutekunst R, Dunn AD. Two simple methods for measuring iodine in urine. Thyroid 1993;3:119-23.

Nguyen Tri Dung (Dept. of Biochem., Hosp. of Endocrinol., Hanoi, Vietnam) and Maurice L. Wellby * (Dept. of Clin. Chem., Queen Elizabeth Hosp., Adelaide, S. Australia 5011; * author for correspondence: fax 618 8222 6032, e-mail wholdback@tgehsmtp.tgeh.sa.gov.au)
Table 1. Effect of temperature on absorbances at 405 nm for
potassium iodate standards.

 Temperature, [degrees]C
I concn.,
[micro]g/L 33-35 28 25 22

0 0.236 0.817 0.984 1.107
 0.245 1.085
20 0.214 0.670 0.737 0.983
 0.204 0.944
50 0.160 0.492 0.566 0.814
 0.148 0.773
100 0.108 0.270 0.341 0.567
 0.104 0.556
150 0.078 0.156 0.195 0.405
 0.071 0.386
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Title Annotation:Technical Briefs
Author:Dung, Nguyen Tri; Wellby, Maurice L.
Publication:Clinical Chemistry
Date:Jun 1, 1997
Words:516
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