Nasal Potential Difference (NPD) is one of the criteria for diagnosis of CF in questionable cases. The aim was to compare the diagnostic outcome of a cohort of patients with questionable CF phenotypes.
Methods: Patients with suspected CF performed sweat test and NPD.
Results: 449 patients underwent both NPD and sweat tests. 246 had sweat < 40, 33 had >60 and 170 had 40-60, 375 had normal NPD and 74 abnormal, 26 of 375 are in the CF registry. Of the 74 diagnosed as abnormal, 21 are in the CF registry; sensitivity of NPD is 44.7% and specificity 86.8% with negative predictive value of 93% This is better than the sweat test which has sensitivity of 45.4%, specificity of 58.1% and negative predictive value of 88.8%. There were 8 patients with a normal sweat test and abnormal NPD of which 5 are registered as CF. There were 83 patients with a positive sweat test and normal NPD of which 5 are in the CF registry; sensitivity of NPD is 50%, specificity is 96.2%, positive predictive value 62.5% and negative predictive value of 93.9%. Of the 170 patients with borderline sweat test, 5 out of 140 (3.6%) with normal NPD are in the registry while of the 31 patients with abnormal NPD, 7 (23%) are in the registry.
Conclusion: In patients with a questionable diagnosis of CF, NPD is very useful and is best at ruling out the diagnosis in the presence of a false positive sweat test.
