Does Integration of Various Ion Channel Measurements Improve Diagnostic Performance in Cystic Fibrosis?
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RATIONALE: The diagnosis of cystic fibrosis (CF) may remain inconclusive despite comprehensive evaluation. OBJECTIVES: Determine whether combined ion channel measurements (C-ICMs) obtained from different end-organ epithelia can help diagnose CF. METHODS: Prospective enrollment of (1) a training sample of 156 non-CF subjects and 107 patients with CF, and (2) a validation cohort of 202 patients with single-organ CF-like phenotypes. All subjects had genotyping, sweat test, and nasal potential difference (NPD). Principal components analysis was applied to derive various candidate C-ICMs by combining sweat chloride plus every one or two combination(s) of four NPD parameters (maximal potential difference [MaxPD], change in potential difference in response to perfusion with amiloride [ΔAmil], change after chloride-free and isoproterenol perfusion [ΔCl-free+Iso], and total change in potential difference [ΔAmil+Cl-free+Iso]). MEASUREMENTS AND MAIN RESULTS: The best of the 10 candidate C-ICMs, which combined sweat chloride and two NPD parameters (ΔCl-free+Iso and ΔAmil+Cl-free+Iso), diagnosed CF in the training sample with 100% sensitivity and specificity (CF cutoff > 0). In the validation cohort, C-ICM was normal in all subjects with normal sweat test and normal/borderline NPD, with the exception of one subject. C-ICM was abnormal in all subjects when the sweat test was abnormal and the NPD was abnormal/borderline, and when the sweat test was borderline and the NPD was abnormal. C-ICM was abnormal in 75 and 85.7% of subjects with normal sweat chloride plus abnormal NPD, and those with abnormal sweat test plus normal NPD, respectively. In borderline sweat test cases, 23.5, 90, and 100% of subjects had abnormal C-ICM with normal, borderline, and abnormal NPD, respectively. CONCLUSIONS: The concept of combining different measures of cystic fibrosis transmembrane conductance regulator function into a single composite score is feasible. The C-ICM may be useful for diagnostic determination of patients with questionable CF.
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