Sweat chloride (Cl ) concentration is the gold standard for diagnosing cystic fibrosis (CF). This is however, challenging among patients with borderline values. Previous studies have reported that the sweat Cl /Na ratio may be useful for diagnosing CF; however, little is known about Cl /K and (Cl +Na )/K ratios. This study aimed to retrospectively define the most appropriate outcome of the sweat test. Samples of sweat were collected using the Gibson and Cooke method. Cl , Na , and K were further quantified in 2084 participants-1283 CF and 801 non-CF-based on clinical diagnosis. Among those with borderline sweat Cl values (n=502), 34.8% had CF. In the receiver operating characteristic curve analysis, the area under the curve was calculated to evaluate the diagnostic value of the ion ratios. In the overall population, all the ratios significantly discriminated CF from non-CF, whereas in the borderline group, only Cl /Na significantly discriminated CF and non-CF subjects, regardless of age. This article is protected by copyright. All rights reserved.This article is protected by copyright. All rights reserved.