Sweat conductivity: an accurate diagnostic test for cystic fibrosis?
BACKGROUND
Sweat chloride test is the gold standard test for cystic fibrosis (CF) diagnosis. Sweat conductivity is widely used although still considered a screening test.


METHODS
This was a prospective, cross-sectional, diagnostic research conducted at the laboratory of the Instituto da Criança of the Hospital das Clínicas, São Paulo, Brazil. Sweat chloride (quantitative pilocarpine iontophoresis) and sweat conductivity tests were simultaneously performed in patients referred for a sweat test between March 2007 and October 2008. Conductivity and chloride cut-off values used to rule out or diagnose CF were <75 and ≥90 mmol/L and <60 and ≥60 mmol/L, respectively. The ROC curve method was used to calculate the sensitivity, specificity, positive (PPV) and negative predictive value (NPV), as well as the respective 95% confidence intervals and to calculate the area under the curve for both tests. The kappa coefficient was used to evaluate agreement between the tests.


RESULTS
Both tests were performed in 738 children, and CF was ruled out in 714 subjects; the median sweat chloride and conductivity values were 11 and 25 mmol/L in these populations, respectively. Twenty-four patients who had received a diagnosis of CF presented median sweat chloride and conductivity values of 87 and 103 mmol/L, respectively. Conductivity values above 90 mmol/L had 83.3% sensitivity, 99.7% specificity, 90.9% PPV and 99.4% NPV to diagnose CF. The best conductivity cut-off value to exclude CF was <75 mmol/L. Good agreement was observed between the tests (kappa: 0.934).


CONCLUSIONS
The sweat conductivity test yielded a high degree of diagnostic accuracy and it showed good agreement with sweat chloride. We suggest that it should play a role as a diagnostic test for CF in the near future.
Pediatric Pulmonology Division, Instituto da Criança, Hospital das Clínicas, University of São Paulo, Av. Dr. Enéas Carvalho de Aguiar, 647, cep 05403-900 São Paulo - SP - Brazil. Electronic address: anacmattar@uol.com.br.
Ana Claudia Veras
AC
Mattar
Department of Maternal and Child Health, College of Public Health, University of Sao Paulo, Av. Dr. Arnaldo, 715, cep 01246-904 São Paulo - SP - Brazil. Electronic address: leone.claudio@gmail.com.
Claudio
C
Leone
Pediatric Pulmonology Division, Instituto da Criança, Hospital das Clínicas, University of São Paulo, Av. Dr. Enéas Carvalho de Aguiar, 647, cep 05403-900 São Paulo - SP - Brazil. Electronic address: jocarod@uol.com.br.
Joaquim Carlos
JC
Rodrigues
Pediatric Pulmonology Division, Instituto da Criança, Hospital das Clínicas, University of São Paulo, Av. Dr. Enéas Carvalho de Aguiar, 647, cep 05403-900 São Paulo - SP - Brazil. Electronic address: fabiola.adde@outlook.com.
Fabíola Villac
FV
Adde
D002675:Child, Preschool; D002712:Chlorides / Q000032:analysis*; D003430:Cross-Sectional Studies; D003550:Cystic Fibrosis / Q000175:diagnosis*; D004553:Electric Conductivity; D005260:Female; D006801:Humans; D008297:Male; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D013542:Sweat / Q000737:chemistry*
D002712:Chlorides
Conductivity; Cystic fibrosis; Pilocarpine; Sweat test