Sweat bromide excretion in cystic fibrosis

Sweat reabsorption of bromide and chloride was studied in controls, obligate carriers of the cystic fibrosis gene, and individuals with cystic fibrosis. Sweat and serum ultrafiltrate concentrations of bromide and chloride were determined by an anion-exchange chromatographic method. The reabsorption of each ion was determined by the ratio of the sweat concentration to the serum ultrafiltrate concentration. Sweat bromide reabsorption paralleled chloride reabsorption in each of the groups. The mean bromide reabsorption ratio for 12 controls was 0.10 +/- 0.10 (range 0 to 0.26), for 11 obligate carriers 0.09 +/- 0.09 (range 0 to 0.24), and for 13 individuals with cystic fibrosis 0.86 +/- 0.26 (range 0.47 to 1.38). The mean chloride reabsorption ratio for the controls was 0.24 +/- 0.11 (range 0.06 to 0.45), for the obligate carriers 0.22 +/- 0.10 (range 0.10 to 0.45), and for individuals with cystic fibrosis 0.95 +/- 0.09 (range 0.76 to 1.1). Thus, there was a significant decrease in bromide reabsorption in individuals with cystic fibrosis compared with controls (P less than 0.001) and heterozygotes (P less than 0.001), which was similar for chloride. As with chloride reabsorption, no difference was found in bromide reabsorption between obligate carriers and controls, indicating that this measurement could not be used for heterozygote detection. In each of the three groups bromide reabsorption was greater than chloride reabsorption. This finding is consistent with a size-dependent anion channel in the sweat epithelial cell because bromide has a smaller hydrated radius than chloride.