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Sweat conductivity for diagnosing cystic fibrosis after positive newborn screening: prospective, diagnostic test accuracy study
  1. Renata Marcos Bedran1,
  2. Cristina Gonçalves Alvim1,
  3. Olívia Gonçalves Sader1,
  4. José Vicente Alves Júnior2,
  5. Fernando Henrique Pereira2,
  6. Daniela Magalhães Nolasco2,
  7. Linjie Zhang3,
  8. Paulo Camargos1
  1. 1Faculty of Medicine, Department of Pediatrics, and Cystic Fibrosis Clinic, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil
  2. 2Centre for Newborn Screening and Genetic Diagnosis, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil
  3. 3Faculty of Medicine, Federal University of Rio Grande, Rio Grande, Rio Grande do Sul, Brazil
  1. Correspondence to Professor Paulo Camargos, Professor Paulo Camargos, Faculty of Medicine, Department of Pediatrics, and Cysti Fibrosis Clinic, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil; pcamargs{at}


Objective To assess the accuracy of sweat conductivity among newborns and very young infants.

Design Prospective, population-based, diagnostic test accuracy study.

Setting Public Statewide Newborn Screening Programme where the incidence rate of cystic fibrosis (CF) is ≈1:11 000.

Patients Newborns and very young infants with positive two-tiered immunoreactive trypsinogen.

Interventions Sweat conductivity and sweat chloride were performed simultaneously, on the same day and facility by independent technicians, with the cut-off values of 80 mmol/L and 60 mmol/L, respectively.

Main outcome measures Sensitivity, specificity, positive and negative predictive values (PPV and NPV), overall accuracy, positive and negative likelihood ratios (+LR, -LR) and post (sweat conductivity (SC)) test probability were calculated to assess SC performance.

Results 1193 participants were included, 68 with and 1108 without CF, and 17 with intermediate values. The mean (SD) age was 48 (19.2) days, ranging from 15 to 90 days. SC yielded sensitivity of 98.5% (95% CI 95.7 to 100), specificity of 99.9% (95% CI 99.7 to 100), PPV of 98.5% (95% CI 95.7 to 100) and NPV of 99.9% (95% CI 99.7 to 100), overall accuracy of 99.8% (95% CI 99.6 to 100), +LR of 1091.7 (95% CI 153.8 to 7744.9) and -LR of 0.01 (95% CI 0.00 to 0.10). After a positive and negative sweat conductivity result, the patient’s probability of CF increases around 350 times and drops to virtually zero, respectively.

Conclusion Sweat conductivity had excellent accuracy in ruling in or ruling out CF after positive two-tiered immunoreactive trypsinogen among newborns and very young infants.

  • cystic fibrosis
  • child health services

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  • Contributors RMB verified the underlying data, contributed to the conception, writing, revision and editing of the manuscript. CGA contributed to the manuscript’s conception, writing, editing and critical revision. JVAJ and FHP contributed to data collection, analysis, validation and interpretation, verified the underlying data and contributed to writing the manuscript. LZ contributed to the manuscript’s conception, writing, revision and editing. PC conceived the study and its design, had primary responsibility for protocol development, contributed to the investigation, methodology, project administration, validation and verified the underlying data; conception, writing, editing and revising of the manuscript. PC is the guarantor. All authors confirm that they had full access to all the data in the study and accept responsibility for submitting it for publication.

  • Funding This work was supported by the Brazilian Council for Research and Technological Development, grant number 486201/2013-9, to PC.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.