Sweat conductivity test - can it be a cheaper alternative to sweat chloride analysis for diagnosis of cystic fibrosis in low resource setting?

Background & objectives Availability of sweat chloride analysis, the gold standard test for diagnosis of Cystic Fibrosis (CF) faces significant challenges in India. This study aimed to compare sweat conductivity using Sweat-Chek™ Sweat Analyzer against sweat chloride analysis using the 926 Sherwood chloride analyser and assess if sweat conductivity test can guide CF diagnosis in resource-poor settings. Methods In this retrospective study sweat chloride analysis and sweat conductivity were simultaneously performed on samples collected via Macroduct® system from patients referred for sweat testing. CF diagnosis was based on sweat chloride levels: ≥60 mmol/l confirmed CF, 30-59 mmol/l was borderline, and <30 mmol/l excluded CF. Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV), and area under curve (AUC) were calculated via ROC curve. Spearman's rho was employed to analyse the correlation between methods. Results Both tests were performed on 118 children of which 106 samples were adequately collected. CF was diagnosed in 11 children. Sweat conductivity ≥ 80 mmol/l diagnosed CF with 100 per cent sensitivity, specificity, PPV, and NPV. Likewise, a value ≤ 49 mmol/l predicted absence of CF with 100 per cent sensitivity, 91.36 per cent specificity, 78.13 per cent PPV, and 100 per cent NPV. Spearman's rho of 0.93 (P< 0.001) showed a strong correlation between the two methods. Intermediate conductivity values also correlated well (rs 0.62, P< 0.003) with intermediate sweat chloride levels. Interpretations & conclusions Sweat conductivity reliably identified CF in the study population including those children with borderline levels, suggesting the possibility of its use in resource-limited settings where sweat chloride analyzers are unavailable.