Abstract
Dear Sir,
It is now 26 years since the introduction of the Wescor Macroduct system for the collection of sweat and measurement of conductivity. The collection system has been widely adopted due to its ease of use and maintenance. Although conductivity measurement is simple and robust it has not been so widely adopted as conductivity measurements alone are not recommended for diagnosis of cystic fibrosis in the UK 1 or USA. 2 A recent report in this journal 3 highlights the use of conductivity as a screening test in the Australian guidelines; any patient with a sweat conductivity result greater than 50 mmol/L NaCl eq. should be followed up with chloride measurement, either on the existing sweat sample or on a fresh collection.
Over the years many groups have attempted to compare the diagnostic performance of sweat conductivity and chloride concentrations. We have collected comparison data over two years which is shown in Figure 1; all samples with detectable sweat chloride concentrations, including repeat samples, are included. Our data are remarkably similar to that previously published; showing closer agreement of conductivity and chloride results in the cystic fibrosis patients compared with the unaffected patients (Figure 1a)
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and a second-order polynomial giving a line of best fit
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(Figure 1b). In common with others we have very few results in the intermediate region. The only sample with an intermediate chloride concentration was from the unaffected sibling of a cystic fibrosis patient; genetic testing has not been performed to define carrier status. Genetic testing of the other patients with intermediate conductivity results failed to identify any of the 32 most common cystic fibrosis mutations. The reluctance in the literature to use conductivity measurements for diagnosis of cystic fibrosis is based on sensitivity and specificity calculations. However, the fact that conductivity and chloride measurements may differ in their ability to identify individual patients in the intermediate range is only to be expected. These patients are comparatively rare and will always pose a diagnostic challenge even when using sweat chloride quantitation. The diagnosis of cystic fibrosis in any patient will involve clinical examination and genetics as well as biochemistry.
(a) Difference plot showing agreement of chloride and conductivity results. (b) Relationship between sweat chloride concentration and conductivity in 155 patient samples. Vertical and horizontal dashed lines indicate the cut-offs between normal, intermediate and cystic fibrosis results as given in the UK guidelines. Points in bold indicate results that fell above the normal range with linking lines to show repeat sweat collections.
Chloride measurements may be subject to pipetting, weighing and calculation errors as methods are adapted to handling very small samples. The sample is usually consumed during chloride analysis. In contrast, the Wescor SweatChek analyser is designed to measure the conductivity of small sweat samples directly without dilution and the sample may be recovered after analysis. It is our view that sweat conductivity can play a role in the diagnosis of cystic fibrosis and its measurement should be incorporated into the next version of the UK guidelines.
DECLARATIONS