The evaluation of a novel conductometric device for the diagnosis of cystic fibrosis.

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.