Thirty Years of Sweat Chloride Testing at One Referral Center

Abstract

Objective: To conduct a descriptive analysis of the sweat test (ST), associating ST results with epidemiological data, CFTR (cystic fibrosis transmembrane conductance regulator) mutations and reasons to indicate the ST, as well as correlating sweat sodium and sweat chloride concentrations in subjects.

Methods: Retrospective survey and descriptive analysis of 5,721 ST at a university referral center.

Results: The inclusion of the subjects was based on clinical data related with cystic fibrosis (CF) phenotype. The samples were grouped by (i) sweat chloride concentrations (mEq/L): <30: 3,249/5,277 (61.6%); ≥30 to <60: 1,326/5,277 (25.1%); ≥60: 702/5,277 (13.3%) and (ii) age: (Group A--GA) 0 to <6 months; (Group B--GB) ≥6 months to <18 years; (Group C--GC) ≥18 years. Digestive symptoms showed higher prevalence ratio for the CF diagnosis as well as association between younger age and higher values of sweat chloride, sweat sodium, and chloride/sodium ratio. The indication of ST due to respiratory symptoms was higher in GB and associated with greater age, lower values of sweat chloride, sweat sodium, and chloride/sodium ratio. There was higher prevalence of ST with sweat chloride levels <30 mEq/L in GB, ≥60 mEq/L in GC, and with borderline level in GB. There was positive correlation between sweat sodium and sweat chloride. Sweat chloride/sweat sodium and sweat sodium-sweat chloride indexes showed association with sex, reason for ST indication, and CFTR mutations. Sex alters some values presented in the ST. The number of ST/year performed before and after the newborn screening implementation was the same; however, we observed a higher number of borderlines values. A wide spectrum of CFTR mutation was found. Severe CFTR mutations and F508del/F508del genotype were associated with highest probability of ST chloride levels ≥60 mEq/L, and the absence of CFTR mutations identified was associated with borderline ST and respiratory symptoms.

Conclusions: ST data showed wide variability dependent on age, sex, reason for examination indication, CFTR mutations, and weight of the collected sweat sample. Sweat sodium concentration is directly correlated with sweat chloride levels and it could be used as a quality parameter.

Keywords: cystic fibrosis; diagnosis; sweat chloride; sweat sodium; sweat test.

Figure 1

Association between the ST results considering chloride cutoff values and sweat weight (mg), sodium value (mEq/L), and subject’s age (years). (A) Sweat weight: (chloride <30 mEq/L) N = 3,249; mean of 175 ± 43; median of 172 (amplitude from 75 to 367); (chloride ≥30 to <60 mEq/L) N = 1,326; mean of 162 ± 41; median of 154 (amplitude from 78 to 299); (chloride ≥60 mEq/L) N = 702; mean of 164 ± 42; median of 159 (amplitude from 78 to 326). Chloride <30 mEq/L ≠ chloride ≥30 to <60 mEq/L, and chloride ≥60 mEq/L. (B) Sweat–sodium concentration: (chloride <30 mEq/L) N = 3,249; mean of 25.24 ± 7.03; median of 24.4 (amplitude from 10 to 68.5); (chloride ≥30 to <60 mEq/L) N = 1,326; mean of 45.82 ± 9.89; median of 45.11 (amplitude from 20.3 to 81.8); (chloride ≥60 mEq/L) N = 702; mean of 80.54 ± 17.98; median of 77 (amplitude from 42.9 to 149.7). All groups differ from each other. (C) Subject’s age: (<30 mEq/L) N = 3.233; mean of 6.66 ± 11.91; median of 2.17 (amplitude from 0 to 76.33); (chloride ≥30 to <60 mEq/L) N = 1,282; mean of 24.34 ± 21.74; median of 15 (amplitude from 0 to 85.58); (chloride ≥60 mEq/L) N = 621; mean of 21.27 ± 22.52; median of 12.25 (amplitude from 0 to 81.17). All groups differ from each other. Statistical analysis was performed by Kruskal–Wallis test. P-value for all analyses was <0.001. α = 0.05. The median is set in red marker and the 95% confidence interval is set in green line. mEq/L, milliequivalents per liter; ST, sweat test.

Figure 2

Correlation between sweat weight and sweat chloride and sodium concentrations, subject’s age, difference between sweat chloride and sodium concentrations, and sodium/chloride ratio. (A) Correlation between sweat weight and chloride concentrations. N = 5,277; rho = −0.201 (95% CI = −0.227 to −0.175), p < 0.0001. (B) Correlation between sweat weight and sweat sodium concentrations. N = 5,277; rho = −0.257 (95% CI = −0.282 to −0.232), p < 0.001. (C) Correlation between sweat weight and subject’s age. N = 5,136; rho = –0.075 (95% CI = –0.102 to –0.048), p < 0.0001. (D) Correlation between sweat weight and the difference between sweat sodium and chloride concentrations. N = 5,277; rho = –0.094 (95% CI = –0.121 to 0.067), p < 0.0001. (E) Correlation between sweat weight and chloride/sodium ratio. N = 5,277; rho = 0.007 (95% CI = –0.02 to 0.034), p = 0.621. The statistical analysis was performed by Spearman’s rank correlation test. α = 0.05. 95% CI, confidence interval of 95%.

Figure 3

Distribution of sex of subject, considering sweat chloride (mEq/L) and sweat weight. (A) Sweat weight (mg). (chloride <30 mEq/L) Female: N = 1,415; mean of 171 ± 43; median of 167 (amplitude from 75 to 337); male: N = 1,821; mean of 178 ± 43; median of 175 (amplitude from 76 to 367) (p < 0.001); (chloride ≥30 to <60 mEq/L) female: N = 691; mean of 156 ± 38; median of 149 (amplitude from 87 to 279); male: N = 633; mean of 168 ± 44; median of 162 (amplitude from 78 a 299) (p < 0.001); (chloride ≥60 mEq/L) Female: N = 370; mean of 157 ± 39; median of 150 (amplitude from 78 to 295); male: N = 332; mean of 170 ± 44; median of 168 (amplitude from 79 to 326) (p < 0.001). (B) Sweat chloride concentrations obtained in the sweat test. (<30 mEq/L) Female: N = 1,821; mean of 18.15 ± 5.26; median of 17.4 (amplitude from 10 to 29.9); male: N = 1,415; mean of 18.54 ± 5.29; median of 17.7 (amplitude from 10 to 29.9) (p = 0.038); (chloride ≥30 to < 60 mEq/L) female: N = 691; mean of 42.08 ± 8.29; median of 41.1 (amplitude from 30 to 59.8); male: N = 633; mean of 40.89 ± 8.28; median of 39.3 (amplitude from 30 to 59.9) (p = 0.006); (chloride ≥60 mEq/L) female: N = 370; mean of 91.87 ± 24.53; median of 86.85 (amplitude from 60 to 158.6); male: N = 332; mean of 93.28 ± 25.06; median of 91.2 (amplitude from 60 to 159.2) (p = 0.618). The statistical analysis was performed by Mann–Whitney test. α = 0.05. The median is set in red marker and the 95% confidence interval is set in green line. The second graphic is a box plot. mEq/L, milliequivalents per liter.

Figure 4

Regression and correlation of sweat sodium and chloride concentrations distributed according to the diagnostic results considering the sweat test values (total sample). Linear regression: N = 5,277; R² = 0.877; y = −11.05 + 1.1932x (p < 0.001). Correlation: N = 5,277; correlation coefficient r = 0.9365; 95% CI = 0.9331–0.9397 (p < 0.001); (1) N = 3,249; R² = 0.4347; y = 5.8091 + 0.4955x (p < 0.001); (2) N = 1,324; R² = 0.4913; y = 14.5228 + 0.5888x (p < 0.001); (3) N = 702; R² = 0.5336; y = 11.4797 + 1.0064x (p < 0.001). (i) chloride <30 mEq/L; (ii) chloride ≥30 to <60 mEq/L; and (iii) chloride ≥60 mEq/L. α = 0.05. mEq/L, milliequivalents per liter; 95% CI, confidence interval of 95%.

Figure 5

The normal variability enrolled with the sweat test, which could direct the diagnosis to cystic fibrosis, mainly in the limits (borderline zones 1 and 2) associated with the sweat test cutoff points. In addition, sex, CFTR mutation, sweat weight, digestive symptoms, pulmonary symptoms, and neonatal screening influenced the values achieved in the sweat test, and this influence is present in all possible sweat test classes: (i) <30 mEq/L; (ii) ≥30 to <60 mEq/L; and (iii) ≥60 mEq/L. CFTR, cystic fibrosis transmembrane regulator.