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Background: Multiple breath washout (MBW) is an informative but time-consuming test. This study evaluates the uncertainty of a time-saving predictor algorithm in adolescents. Methods: Adolescents were recruited from the Copenhagen Prospective Study on Asthma in Childhood (COPSAC 2000 ) birth cohort. MBW trials were performed at 13 y of age with Innocor model Inn00400 using sulfur hexafluoride (SF 6 ) as tracer gas. Measurements were analyzed using a mixed model focusing on two prediction points doubling (t5%) and quadrupling (t10%) the standard end point (t2.5%). results: One hundred and seventy-two MBW trials conducted in 78 adolescents with and without asthma from COPSAC 2000 were included. At t10%, the washout time (WoT) was reduced by 41%, and an uncertainty of 0.159 lung clearance index (LCI) units was introduced (±2 SD), ±1.27). At t5%, the WoT was reduced by 25%, with an uncertainty of 0.083 LCI units (±0.558). The optimal prediction point, which led to most saved time and least uncertainty was t5%. conclusion: The predictor algorithm is capable of shortening the MBW test time but introduces an increasing uncertainty with earlier prediction points. This first-of-a-kind prediction algorithm holds promise in shortening the MBW test in children but should be used with caution in subjects with normal LCI values.