Introduction: Chronic rejection (CLAD) after lung transplantation (LTx) is identified by a drop in baseline spirometry. Lung clearance Index (LCI) derived from multiple breath washout (MBW) is more sensitive than spirometry at detecting early signs of small airway disease. LCI is raised in a significant percentage of LTx recipients in stable patients with a normal FEV₁. However, it is unclear whether this measurement is repeatable and if baseline LCI values change over time in lung transplant recipients. 

Methods: MBW was performed at clinic visits in double LTx recipients, using a closed circuit Innocor MBW system. Patients were recruited between 6 months and 6 years post-LTx and were CLAD-free at time of recruitment. Those with >12months of follow-up data, more than one measurement and who did not develop CLAD have been included.

Results:  24/40 patients had multiple measurements in a 12 month period. Seven of these patients were excluded due to CLAD- leaving 17 patients in the entire cohort. A median of 5 LCI measurements per participant was obtained. LCI change from baseline in this cohort was -0.97% Median (IQR) LCI in this group was 7.10, (-12.98-12.18). Inter-visit variability was also acceptable, with mean CoV 4.40%.

Conclusions: Longitudinal measurements of MBW are feasible and repeatable in CLAD-free lung transplant recipients, revealing stable LCI. The variation is comparable to other clinical populations and therefore this may be a useful measure in Lung transplantation. Ongoing work will establish if changes over time in small airway indices precede sustained drops in FEV₁, facilitate earlier detection of CLAD and help differentiate phenotypes.