Background: We evaluated the diagnostic discrimination of a new micro-flow cell device (Nanoduct®) which measures sweat conductivity in situ at a regional referral centre for cystic fibrosis (CF).
Methods: The device was evaluated in comparison to the measurement of sweat chloride with the established quantitative pilocarpine iontophoresis test (QPIT) and extended in a number of patients to conductivity measurements in liquid sweat collected with the Macroduct® system. Sweat testing was conducted simultaneously on patients referred for diagnostic sweat testing, on patients known to have CF and on adult volunteers. The intra-individual variability, the failure rate and diagnostic accuracy were determined.
Results: A total of 110 tests were performed on 100 individuals, 36 of whom had classical CF and six of whom had non-classical CF. The Nanoduct system produced a false negative result in one quarter of the patients with classical CF. Moreover, conductivity was negatively biased compared with chloride in this group. Repeat testing of the false negatives using a new batch of sensors and/or measuring conductivity in liquid sweat collected with the Macroduct device gave accurate diagnostic discrimination indicating that the original sensors were faulty. Photographic examination confirmed that a batch of sensors were defective.
Conclusions: Our experience suggests that the prototype microflow cell conductometric device cannot be used for the diagnosis of CF due to the high false negative rate. As a consequence of this study, the manufacturers have implemented a pre-testing system to quality control the sensors prior to issue.
