Uncertainty of sweat chloride testing: does the right hand know what the left hand is doing?

Ann Clin Biochem 2008;45:535-538
doi:10.1258/acb.2008.008127
© 2008 Association for Clinical Biochemistry

 

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R J Mackay,
C M Florkowski,
P M George,
C W Sies and
S Woods


Biochemistry Unit, Canterbury Health Laboratories, PO Box 151, Christchurch 8011, New Zealand


Corresponding author: Dr R J Mackay. Email: richard.mackay{at}cdhb.govt.nz

Although analytical variation in sweat electrolyte testing can be easily estimated, there is limited data on total variation. This study aims to evaluate the total variation of the sweat test by measuring the difference between sweat electrolyte values in specimens obtained simultaneously from two sites. Chloride is recommended in published guidelines as the only discriminant for the diagnosis of cystic fibrosis, and sodium may be measured as a guide to the adequacy of collection and analysis. Both are reported here. Sweat was collected in patients by the Gibson Cooke method from two sites simultaneously. Coefficient of variation in this laboratory is 4.1 and 5% for chloride and sodium, respectively. 295 patients had sufficient sweat collected from both sites for analysis. The values for chloride and sodium were compared between the two sites. The total coefficient of variation (CVt)calculated for the whole group between the two sites was 20.2%for chloride and 16.9% for sodium, and the standard deviations4.3 mmol/L and 4.8 mmol/L, respectively. In patients with intermediatechloride concentrations; in different age groups; and when thosetests with a difference between sodium and chloride concentrationof more than 15 were excluded, minimal differences in thesefigures were observed. Use of strictly defined cut-off pointsto discriminate between normal and intermediate electrolytevalues, and between intermediate and raised electrolyte values,does not reflect the variation in sweat electrolyte contentfound within an individual patient. This has important implicationsfor reporting.

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