Conductivity Found To Be a Reliable Test for Cystic Fibrosis

Accuracy and efficiency are of utmost importance when deciding on a diagnostic tool to detect cystic fibrosis in newborns. The gold standard of detection is measuring sweat ion concentration, as patients with defect CFTR proteins have an elevated electrolyte content in their sweat. However, another method that is now only used to screen for cystic fibrosis may hold greater diagnostic power than previously thought. Researchers from Neonatal Screening Reference Service in Brazil determined that the Sweat Check 3120^®, which is a conductivity measuring device produced by Wescor Inc. in Utah, can lead to diagnoses of cystic fibrosis that agree with the quantitative sweat ion tests.

An ionic test is considered positive if chloride concentration in sweat is greater than 60 millimoles per liter of sweat. In the case of newborns, concentrations between 30 and 50 millimoles per liter are highly suggestive of cystic fibrosis. A conductivity test is considered a positive screen if values are greater than 80 millimoles per liter of sweat, but are only suggestive of further screening if they are between 50 and 80 millimoles per liter.

Two methods of detection were compared head-to-head in the study, “Sweat Conductivity and Coulometric Quantitative Test in Neonatal Cystic Fibrosis Screening,” which was published in Jornal de Pediatria. Over 2,000 newborns with cystic fibrosis had their sweat ion concentration measured using conductivity and coulometry and were included in the final results. Patients were selected when they had an initial screening of greater than 70 grams per liter of sweat. Of these patients, 90 cases of cystic fibrosis were confirmed.

Researchers on two different teams conducted the testing and were unaware of the others’ results, so as to not influence their own interpretation of the measurements. While conducting the tests, the conductivity test was comparable to the coulometric test, which is the gold standard. Since 100% of cystic fibrosis cases were detected with conductivity, the test had a sensitivity of 100%.

“The results of this study suggest that the analytical method of conductivity can be a definitive and reliable diagnostic procedure for cystic fibrosis,” wrote the authors. Therefore, conductivity may be a valid test for clinicians to use to diagnose patients with cystic fibrosis.