Intestinal current measurement versus nasal potential difference measurements for diagnosis of cystic fibrosis: a case-control study

Background: Nasal potential difference (NPD) and intestinal current measurement (ICM) are functional CFTR tests that are used as adjunctive diagnostic tools for cystic fibrosis (CF). Smoking has a systemic negative impact on CFTR function. A diagnostic comparison between NPD and ICM and the impact of smoking on both CFTR tests has not been done. Methods: The sweat chloride test, NPD, and ICM were performed in 18 patients with CF (sweat chloride >60 mmol/l), including 6 pancreatic sufficient (PS) patients, and 13 healthy controls, including 8 smokers. The NPD CFTR response to Cl-free and isoproterenol perfusion (Delta 0Cl(-) + Iso) was compared to the ICM CFTR response to forskolin/IBMX, carbachol, and histamine (Delta I-sc, forskolin/IBMX+ carbachol+histamine). Results: The mean NPD CFTR response and ICM CFTR response between patients with CF and healthy controls was significantly different (p < 0.001), but not between patients with CF who were PS and those who were pancreatic insufficient (PI). Smokers have a decreased CFTR response measured by NPD (p = 0.049). For ICM there is a trend towards decreased CFTR response (NS). Three healthy control smokers had NPD responses within the CF-range. In contrast to NPD, there was no overlap of the ICM response between patients with CF and controls. Conclusions: ICM is superior to NPD in distinguishing between patients with CF who have a sweat chloride >60 mmol/l and healthy controls, including smokers. Neither NPD nor ICM differentiated between patients with CF who were PS from those who were PI. Smoking has a negative impact on CFTR function in healthy controls measured by NPD and challenges the diagnostic interpretation of NPD, but not ICM.