Cross-contaminator exclusion test for dry-ashing determination of protein-bound iodine.

XIDATION OF ORGANIC MATERIAL as a preliminary to determination of iodine, whether by wet or dry methods, entails loss of iodine if insufficient organic matter is present during the reaction (1, 2, 3). In blood serum, the organic content suffices to prevent this loss when the amount of iodine in the specimen is in the range found in physiological or pathological sera, 0-30 pg./100 ml., as has been repeatedly shown in recovery experiments (4, 5, 6, 7). When the iodine content of serum exceeds 500 ig./100 ml., as is sometimes the case after administration of iodinated radiopaque media, or other contact with iodinated medication, iodine may be volatilized in the ashing stage of the analysis, to be reabsorbed in other tubes. Such cross-contamination invalidates the entire series of tests, with loss of analyst’s time and delay of reports. To avoid delays caused by the presence of such cross-contaminators, a simple method of screening them out in advance was desirable. In connection with the quantitative dry-ashing procedure in use in this laboratory (7), the cross-contaminator exclusion test described below has proven satisfactory. It requires only 1 drop of serum, and adds no more than 30 minutes of operating time to a series of 20-40 PBI determinations.