The significance of sweat Cl/Na ratio in patients with borderline sweat test

Recently a few cystic fibrosis (CF) patients with borderline or normal sweat tests have been reported. These patients present a diagnostic challenge. We aimed to study the sweat CINa ratio in cystic fibrosis patients and to assess whether this ratio could be used as a diagnostic criteria. The mean sweat CINa ratio of 3 groups was compared: Group A: 71 CF patients carrying 2 mutations known to be associated with severe disease presentation (ΔF508, W1282X, G542X, N1303K, 1717‐1G → A). Group B: 10 compound heterozygous patients who carry one mutation associated with mild clinical disease (3849 + 10 kb C → T). Group C: 142 normal subjects. Sweat chloride levels higher than those of sodium were found in 96% of patients in Group A as compared to 3% of patients in Group C. In Group B 40% of the patients had sweat chloride levels higher than or equal to sodium levels. The mean Cl/Na ratio of Group A (1.2 ± 0.1) differed significantly from that of Group B (0.94 ± 0.1) and both groups had significant higher mean CINa ratio compared to Group C (0.7 ± 0.4) (P < 0.001). Thus in individuals with a borderline sweat test and a Cl/Na ratio < 1 the diagnosis of CF should be considered. However, a Cl/Na ratio < 1 does not exclude CF, since patients carrying mild mutations may have sweat sodium levels higher than those of chloride. Our findings suggest that the sweat CINa ratio in CF is genetically determined and it may be of help in establishing the diagnosis of CF in patients with a borderline sweat test. Pediatr Pulmonol. 1995; 20:369–371. © 1995 Wiley‐Liss, Inc.