Phase 3 slope indices:can accurate estimates be obtained from 2 tests?

Introduction: Multiple-breath washout (MBW) has a time-consuming protocol requiring 3 technically acceptable runs. Lung clearance index based on only 2 runs is as robust as that based on 3. 1,2 We assessed whether accurate estimates of conductive and acinar ventilatory heterogeneity (S cond and S acin ) can be obtained from 2 runs. Method: We analysed N 2 MBW data from 119 school-age children (35with CF). All performed N 2 MBW in triplicate with a pre-set target Vt. Volume corrected S cond and S acin from 3 tests (S 3tests ) were compared to that of the first two tests (S 2tests ) using Bland-Altman plots. The mean difference (S diff ) was calculated as meanS 3tests - meanS 2tests . The sensitivity were determined using upper limits of normal (ULN) from our healthy controls. Results: There was no significant difference in S conddiff or S acindiff for the whole population. The mean (95%LA) was 0.0(0.02,-0.02) and 0.0(0.03,-0.03) for S conddiff and S acindiff respectively. There was a statistically significant difference in S acindiff for healthy subjects (mean (95%LA) -0.002(0.02,-0.03, P =0.03). Among the CF patients, 15 had mean S cond3tests above ULN and 15 had mean S cond2tests above ULN (13 patients in common). The sensitivity was identical for S acin . Concordance of classification occurred in 31(88.5%) subjects. C onclusion: Robust estimates of S cond and S acin can be obtained from 2 tests without compromising sensitivity in CF. In healthy subjects, the difference in S acin was ∼2.8% of published normal values 3 and not clinically significant. The shortened protocol promotes the incorporation of MBW in a clinical context. Ref: 1 Yammine et al. Thorax 2013;68:586, 2 Robinson et al. Ped Pulm 2013;48:336, 3 Yammine et al JCF 2014;13:190.