Shortened Lung Clearance Index is a repeatable and sensitive test in children and adults with cystic fibrosis

BACKGROUND: Lung clearance index (LCI) derived from sulfur hexafluoride (SF(6)) multiple breath washout (MBW) is a sensitive measure of lung disease in people with cystic fibrosis (CF). However, it can be time-consuming, limiting its use clinically. AIM: To compare the repeatability, sensitivity and test duration of LCI derived from washout to 1/30th (LCI(1/30)), 1/20th (LCI(1/20)) and 1/10th (LCI(1/10)) to ‘standard’ LCI derived from washout to 1/40th initial concentration (LCI(1/40)). METHODS: Triplicate MBW test results from 30 clinically stable people with CF and 30 healthy controls were analysed retrospectively. MBW tests were performed using 0.2% SF(6) and a modified Innocor device. All LCI end points were calculated using SimpleWashout software. Repeatability was assessed using coefficient of variation (CV%). The proportion of people with CF with and without abnormal LCI and forced expiratory volume in 1 s (FEV(1)) % predicted was compared. Receiver operating characteristic (ROC) curve statistics were calculated. Test duration of all LCI end points was compared using paired t tests. RESULTS: In people with CF, LCI(1/40) CV% (p=0.16), LCI(1/30) CV%(,) (p=0.53), LCI(1/20) CV% (p=0.14) and LCI(1/10) CV% (p=0.25) was not significantly different to controls. The sensitivity of LCI(1/40), LCI(1/30) and LCI(1/20) to the presence of CF was equal (67%). The sensitivity of LCI(1/10) and FEV(1)% predicted was lower (53% and 47% respectively). Area under the ROC curve (95% CI) for LCI(1/40), LCI(1/30), LCI(1/20), LCI(1/10) and FEV(1)% predicted was 0.89 (0.80 to 0.97), 0.87 (0.77 to 0.96), 0.87 (0.78 to 0.96), 0.83 (0.72 to 0.94) and 0.73 (0.60 to 0.86), respectively. Test duration of LCI(1/30), LCI(1/20) and LCI(1/10) was significantly shorter compared with the test duration of LCI(1/40) in people with CF (p<0.0001) equating to a 5%, 9% and 15% time saving, respectively. CONCLUSIONS: In this study, LCI(1/20) was a repeatable and sensitive measure with equal diagnostic performance to LCI(1/40). LCI(1/20) was shorter, potentially offering a more feasible research and clinical measure.