How are diffusing capacity of lung for carbon monoxide (DLCO) test results interpreted?

Updated: May 14, 2020
  • Author: Kevin McCarthy, RPFT; Chief Editor: Nader Kamangar, MD, FACP, FCCP, FCCM  more...
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Because the DLCO is directly proportional to VA (VA is the lung volume after inhalation of the DLCO test gas, based on the size of the breath of test gas and the dilution of the inspired tracer gas). Nonpulmonary processes that reduce the lung volume at full inflation cause reductions in the DLCO. If VA can be assessed accurately, these reductions produce a normal or elevated KCO. Examples of this include lung resection, thoracic cage abnormalities (eg, kyphoscoliosis), and small lungs. DLCO is reduced in pulmonary emphysema. However, because of the poor distribution of the inspired test gas, the VA may grossly underestimate the TLC, and the resultant KCO may be normal. A reduced DLCO and a reduced KCO suggest a true interstitial disease such as pulmonary fibrosis or pulmonary vascular disease. It has demonstrated that in healthy patients, the KCO is increased to above normal levels when the DLCO test is performed at volumes less than the TLC.

The pattern of a low DLCO and a normal KCO may not be sufficient to rule out the presence of parenchymal disease. The works of Johnson [9] and Chinn et al [10] advocate the volume correction of the predicted value for DLCO by using the measured VA to "correct" the predicted DLCO for low or high lung volumes. Further work is warranted, but studies demonstrating the nonlinearity of the relationship between lung volume and DLCO are sufficiently convincing that the practice of interpreting a low DLCO and a normal KCO (previously known as DLCO/VA) as ”normal” is discouraged. The degree of severity of reduction in the diffusing capacity can be assigned according to the following scheme: less than the predicted lower limit of normal but greater than 60% of predicted is mild, between 40% and 60% of predicted is moderate, and less than 40% is severe.

Nonperfusion of ventilated alveoli, such as in pulmonary vascular disease, produces reduction of both the DLCO and the KCO. Anemia produces a virtual reduction in pulmonary capillary blood volume that causes a reduction in DLCO that can be adjusted mathematically for the reduced hemoglobin. The DLCO may be reduced temporarily in a variety of disorders such as pneumonia, interstitial infiltrative disorders, and alveolar proteinosis. The importance of obtaining an inspiratory vital capacity (IVC) greater than 90% of the best measured VC from the day of the test cannot be overemphasized. Inability to achieve an IVC of greater than or equal to 90% of the largest VC measured that day must be noted on the report.

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