Lung clearance index (LCI) has good clinimetric properties and an acceptable feasibility profile as a surrogate end point in cystic fibrosis (CF). Although most studies to date have been in children, increasing numbers of adults with CF also have normal spirometric findings. Further study of LCI as an end point in adults with CF is required. Therefore, the purpose of this study was to determine the clinimetric properties of LCI across the age range of people with CF.
Clinically stable adults and children with CF and age-matched healthy control subjects were recruited.
LCI and spirometry data for 110 subjects with CF and 61 control subjects were collected at a stable visit. The CF Questionnaire-Revised (CFQ-R) was completed by 80 of 110 subjects with CF. Fifty-six subjects with CF completed a second stable visit. The LCI coefficient of variation percent was 4.1% in adults and 6.3% in children with CF. The coefficient of repeatability of LCI was 1.2 in adults and 1.3 in children. In both adults and children, LCI (area under the receiving operator characteristic curve [AUCROC] = 0.93 and 0.84, respectively) had greater combined sensitivity and specificity to discriminate between people with CF and control subjects when compared with FEV1 (AUCROC = 0.88 and 0.60, respectively) and forced expiratory flow at 25% to 75% of the curve (AUCROC = 0.87 and 0.68, respectively). LCI correlated significantly with the CFQ-R treatment burden in adults (r = –0.37; P < .01) and children (r = –0.50; P < .01). Washout tests were successful in 90% of subjects with CF and were perceived as comfortable and easy to perform in both adults and children.
These data support the use of LCI as a surrogate outcome measure in CF clinical trials in adults as well as in children.