The potential of closed circuit lung clearance index (LCI) to provide longitudinal clinical utility in cystic fibrosis (CF)

Background: Closed circuit washin offers a quicker test with the advantages of an exogenous tracer to measure FRC and LCI. We measured LCI in CF paediatric and adult outpatient clinics to assess repeatability of LCI and track changes over time, and in healthy volunteers (HV) to establish normal range. Methods: Washin was by rebreathing through a closed circuit of 0.2% SF6, with washout on air. CF patients were free of chronic Pseudomonas, with FEV1>50% predicted. Paired clinically stable measurements within 6 months were used to assess LCI repeatability. LCI was also measured in HV aged >5yrs, recruited from local schools (and performed on-site), and two clinic sites. Values are mean (SD) unless stated Results: 112 CF children and adults completed 846 LCI measurements (median 7 per subject, range 1-24). At baseline visit, age was 16.6 (10.1) years and FEV1 z score -1.18 (1.28), consistent with mild lung disease. LCI at first visit was 8.0 (1.9), median coefficient of variation (CoV) 3.9%, median total test time 17.7 mins. 80 CF patients contributed 317 valid data pairs to assess repeatability. Mean difference in LCI was 0.02(0.85), limits of agreement of repeat LCI -1.65 to 1.68. 95% of visits had change in LCI of