The diffusing capacity (DLCO) is a test of the integrity of the alveolar-capillary surface area for gas transfer. It may be reduced in disorders that damage the alveolar walls (septa) such as emphysema, which leads to a loss of effective surface area and 'poor gas mixing' which is illustrated by obstruction from Spirometry and large Residual Volumes (trapping) from Plethysmography. The DLCO is also reduced in disorders that thicken or damage the internal structure of the alveolar walls such as pulmonary fibrosis. AlsoThe DLCO is also reduced in conditionsinterstitial whichlung result in alveolaritisdiseases such as allergichypersensitivity reactionspneumonitis (e.g bird fanciers lung, farmers lung) or dust-inhalation diseases such as asbestosis. Because the transfer of CO depends upon how much blood is traversingpresent in the lung capillaries, the DLCO can be reduced due to anemia and raised in polycythemia. Some laboratories adjust or correct the DLCO for a currentserum Heamoglobinhemoglobin to take into account the effect of anemia. Disorders of the pulmonary blood vessels such as chronic thromboembolic disease or pulmonary hypertension also reduce the diffusing capacity, as can any condition/disease which results in ventilation/perfusionreduction mismatching,in althoughpulmonary thecapillary testblood volume. The DLCO is not, however, a sensitive enoughtest for diagnosis ofacute pulmonary embolismsembolism. Cigarette smokers tend to have lower DLCO, which is partly due to damage to the lungs (lowmild grade inflammationemphysema) and partly due to the retention of carbon monoxide from the cigarettes, which causes back pressure (decreasing the CO gradient). For these reasons, the DLCO is a sensitive test for disorders of the lung, but does not point toward a specific diagnosis. By calculating the dilution of the tracer gas, the test can also be used to estimate the [[total lung capacity]]. In the presence of poor gas mixing from obstructive lung disease the Punjabi correction is used to measure the single breath total lung capacity.
