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.
