Measurement of the diffusing capacity of the lung is part of the routine pulmonary function assessment in every patient and the single breath method is the most commonly used method. This method requires a subject to inspire a gas mixture followed by a 10 second (s) breath-hold. However dyspnoea may preclude measurement in patients with advanced pulmonary disease. We sought to determine if breath-hold time reduction had a significant effect on measured DLco values. Forced spirometry and

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... sion by the single breath method were performed by a respiratory scientist with a Jaeger master-screen PFT equipment, in duplicate with breath-holding of 10 , 8 and 6 s in 30 control subjects (FEV 1 107 ± 12.04% predicted), 30 severe COPD patients (FEV 1 37.2 ± 7.92% predicted), and 30 patients with interstitial lung disease (ILD) (FEV 1 69.5 ± 17.61% predicted). There was no significant difference between DLcoSB and KCO measured at 10, 8 and 6 s in the control group (p=0.4431) and ILD group (p=0.5915). However, there was a significant difference between DLcoSB (p=0.0003) and DLco (VA) (p=0.0183) measured at 10, 8 and 6 s in the COPD group. In the presence of severe airway obstruction the DLco decreases with breath-hold time reduction. In the control group and patients with ILD, there was no significant change in the DLcoSB when breath-hold time was reduced from 10 to 6 s. This could allow a reduction in breath-hold time when measuring the DLco in patients with advanced ILD who are unable to breath-hold for 10 s, but not in patients with severe COPD. 3