Carbon monoxide is useful because it binds rapidly to hemoglobin after inhalation. Measuring the amount remaining after a breath-hold provides a sensitive estimate of how effectively gas crosses from the alveoli into the blood. This approach makes the test informative about both the alveolar-capillary membrane and the pulmonary capillary blood volume involved in gas exchange.
A diffusion capacity result reflects two major parts of pulmonary gas transfer: the condition of the alveolar-capillary membrane and the amount of blood available in pulmonary capillaries. Considering both components helps clinicians assess whether abnormal gas exchange may relate to the lung tissue interface, the pulmonary circulation, or contributions from both areas.
The measurement supplies information about gas exchange that complements other clinical assessments. It can contribute to evaluation of interstitial lung disease, emphysema, and pulmonary vascular disorders, each of which may affect the pathway between inhaled gas and the bloodstream. It is also relevant when breathlessness remains unexplained by the initial clinical evaluation.
The patient inhales a small concentration of carbon monoxide and then holds the breath for a specified measurement period before the gas is assessed. The test compares the inhaled carbon monoxide with the amount remaining after the breath-hold. Because the gas binds rapidly to hemoglobin, this measurement provides information about transfer into pulmonary blood.
Clinicians may request diffusion capacity testing when assessing suspected or known interstitial lung disease, emphysema, or pulmonary vascular disorders. It can also be selected for unexplained breathlessness, when information about gas exchange is needed beyond other pulmonary function findings. The result helps characterize pulmonary involvement within a broader clinical evaluation.
Beyond initial evaluation, the measurement can support disease monitoring and assessment of treatment effects by providing information about gas exchange over time. Clinicians may also include it in preoperative evaluation, where understanding pulmonary function contributes to the overall assessment. Its value therefore extends from identifying abnormalities to following clinical status and planning care.