The gas that is used to measure diffusing capacity of lung is:

Carbon monoxide (CO) is used to measure diffusing capacity of the lung. CO is a 'diffusion-limited' gas. CO has high solubility in blood. However, when CO is given by inhalation, it does not remain in aqueous phase. Reason: CO has very high affinity for Hb; it enters red cells and combines with Hb. Due to this, CO content of blood will be high but paial pressure of CO (pCO) will not rise. (Paial pressure is exeed by the gas in solution and not by the gas that combines with Hb). Paial pressure of CO in capillary blood remains close to zero. Diffusing capacity for CO (DLCO) at rest = 25 ml/min/mm Hg.