Biological variation in sweat sodium chloride conductivity

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Abstract
Background Sweat conductivity, which is equivalent to sweat NaCl concentration, is used as a screening test to identify possible cystic fibrosis (CF) patients. No data exist on the biological variation of this variable and the influence it may have on the interpretation of sweat testing. The aim of this study was to determine the components of biological variation for sweat sodium chloride conductivity and to apply biological variation parameters in the interpretation of sweat conductivity. Methods Sweat conductivity was determined once a week for 5 consecutive weeks on 15 healthy volunteers, 20 healthy infants and 20 known CF patients. Results The analytical coefficient of variation (CVA) was 1·15% for the high-level control material, with a value of 123 mmol/L, and 1·32% for the normal-level control material with a value of 40 mmoL/L. The within-subject (CVI) and between-subject (CVG) biological variations were 12·0% and 30·0%, respectively, for healthy controls; 18% and 20% for healthy infants; and 7·3% and 6·5% for CF patients, respectively. Using the CVA, CVG and CVI, the 95% reference ranges were determined for the above-mentioned three groups. The calculated 95% ranges for the healthy babies and CF patients were 18-60 mmoL/L and 96-144 mmoL/L. Conclusions Our data support a decision level of > 60 mmoL/L for confirmatory CF testing. A lower decision level will result in an unacceptable high rate of unnecessary confirmation testing.

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