Background Nasal Potential Difference (NPD) is a biomarker of CFTR activity used to diagnose CF and monitor experimental therapies. Limited studies have been performed to assess agreement between expert readers of NPD interpretation using a scoring algorithm. Methods We developed a standardized scoring algorithm for “interpretability” and “confidence” for PD (potential difference) measures, and sought to determine the degree of agreement on NPD parameters between trained readers. Results There was excellent agreement for interpretability between NPD readers for CF and fair agreement for normal tracings but slight agreement of interpretability in indeterminate tracings. Amongst interpretable tracings, excellent correlation of mean scores for Ringer's Baseline PD, Δamiloride, and ΔCl-free + Isoproterenol was observed. There was slight agreement regarding confidence of the interpretable PD tracings, resulting in divergence of the Ringers and Δamiloride, and ΔCl-free + Isoproterenol PDs between “high” and “low” confidence CF tracings. Conclusion A multi-reader process with adjudication is important for scoring NPDs for diagnosis and in monitoring of CF clinical trials.