Rationale: American Thoracic Society guidelines state that a 10% intersession change in diffusing capacity of the lung (DL_CO) should be considered clinically significant. However, little is known about the short-term intersession variability in DL_CO in untrained subjects or how variability is affected by rigorous external quality control. Obejctives: To characterize the intersession variability of DL_CO and the effect of different quality control methods in untrained individuals without significant lung disease. Methods: Data were pooled from the comparator arms of 14 pre-registration trials of inhaled insulin that included non-smoking diabetic patients without significant lung disease. 699 participants performed repeated DL_CO measurements using a highly standardized technique. 948 participants performed repeated measurements using routine clinical testing. Measurements and Main Results: The mean intersession absolute change in DL_CO using the highly standardized method was 1.45 mL/min/mm Hg (5.64%) compared to 2.49 mL/min/mm Hg (9.52% ) in the routine testing group (p<0.0001 for both absolute and percent difference). The variability in absolute intersession change in DL_COincreased with increasing baseline DL_COvalues, whereas the absolute percent intersession change was stable across baseline values. Depending upon the method, 15.5-35.5% of participants had an intersession change of 10%. A 20% threshold would reduce this percentage of patients to 1-10%. Conclusions: Intersession variability in DL_CO measurement is dependent upon the method of testing used and baseline DL_CO. Using a more liberal threshold to define meaningful intersession change may reduce the misclassification of normal variation as abnormal change.