Introduction: Elexacaftor/Tezacaftor/Ivacaftor (ELX/TEZ/IVA) has proved to improve lung function and quality of life in cystic fibrosis (CF) patients with the F508del mutation. Sweat chloride concentration (SCC) is used as a surrogate marker of drug effectivity. However, it is not clear how well it correlates with clinical treatment outcomes.

Aims and Objectives: We aimed to assess if SCC change in patients on ELX/TEZ/IVA is a good surrogate marker of treatment response.

Methods: We performed a retrospective study of patients on ELX/TEZ/IVA followed at a Portuguese CF Reference Centre from March 2021 to January 2023. Data on forced expiratory volume in the first second (ppFEV1), SCC, Body Mass Index (BMI), pulmonary exacerbations (Pex) and computerized tomography (CT) scans before and after the beginning of therapy was collected. Statistical analysis was undertaken using SPSS IBM®, Version 27.

Results: There were 45 patients on ELX/TEZ/IVA. Mean SCC decreased significantly by 45 mmol/L (p<,001) after the start of therapy. Mean ppFEV1 at 12-24 weeks improved 11,6% compared to baseline ppFEV1 (p<,001). Pex reduced by 79,6%. BMI increased 8% (p<,001) at 24 weeks. There was no correlation between the percentage of SCC reduction and that of ppFEV1 improvement or BMI increase. We found no association between SCC normalization and radiologic improvement or Pex reduction.

Conclusions: In our study, the degree of SCC decrease did not reflect the clinical or radiological changes of patients on ELX/TEZ/IVA. Further studies are necessary to assess the suitability of SCC as a marker of drug effectivity.