24 Direct potentiometric measurement for chloride ion using Spotchem Arkray analyser adapted to sweat samples

Abstract

Objectives: Sweat testing following pilocarpin iontophoresis is the preferred method to check for cystic fibrosis. Correct measurement and interpretation of results are essential components of cystic fibrosis testing. Methods: All sweat tests performed in the years 2005 to 2010 in our cystic fibrosis centre were analyzed retrospectively. Pilocarpin iontophoresis was in accordance to established international guidelines. All samples were analyzed using chloridometry (Chloridmeter, Kreienbaum, Germany) and conductivity measurement (SweatCheck, WesCor Inc., USA). Results were interpreted as negative, indeterminate or positive according to international guidelines (chloridometry) or manufacturer’s recommendation (conductivity measurement). Results: 1246 sweat tests were performed on 1077 patients. 68 patients were 18 years or older. Median age at testing was 1.58 years (min. 14 days, max. 77 years). Comparison of conductivity measurement and chloridometry reveals a constantly higher NaCl equivalent in conductivity measurement (mean value of difference 24.1±10.7mmol/l). In 91.4% the assessment of test results reveals consistent diagnoses. 0.9% of patients would not have received a second test if only conductivity measurement had been used although chloridometry revealed positive or indeterminate results. Discussion: Sweat testing remains a particularly paediatric topic. The gold standard is chloridometry. This study underlines the inappropriate comparability of conductivity measurement and chloridometry with consequently higher values in conductivity measurement. Special reference values allow a consistent interpretation in many cases but not in all.