How is a diffusing capacity of lung for carbon monoxide (DLCO) test performed?

Updated: May 14, 2020
  • Author: Kevin McCarthy, RPFT; Chief Editor: Nader Kamangar, MD, FACP, FCCP, FCCM  more...
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Most pulmonary laboratories perform this test by the single-breath technique (DLCO SB) because it is quicker to perform and more reproducible than other techniques. Other techniques, such as the rebreathing technique, are not commonly available and are not described here. In the single-breath technique, the subject exhales to RV and then inspires the test gas (tracer gas, [commonly either 10% helium or 0.3% methane], 0.3% CO, 21% oxygen, and balance nitrogen) briskly to TLC. This vital capacity–size breath is held for 10 seconds and then exhaled either into a sample bag (discrete sampling) or past a sampling port leading to rapid-response analyzers after an initial discard of 0.75-1 L of the exhalate to minimize the contribution of dead space gas (mouthpiece, filter, measuring equipment, and anatomical areas where no gas exchange is expected) to the gas sample that will be analyzed to estimate uptake of CO by the alveolar capillaries. The grab sample (0.75-1 L) then is analyzed for tracer gas and CO. The dilution of the tracer gas in the vital capacity–size breath of test gas by the patient's RV provides both a means to estimate the initial alveolar concentration of CO and to estimate the patient's lung volume at full inflation. The rate of diffusion of the CO can be estimated by the change from this initial alveolar concentration to that of the expired grab sample. This change in the CO concentration is then multiplied by the single-breath estimate of TLC to calculate the diffusing capacity. Abnormal hemoglobin (Hb) levels can affect the diffusing capacity and, if known, should be used to mathematically correct the measured diffusing capacity to what it would be if the patient’s hemoglobin was normal. Although it has been recommended that the predicted value be adjusted for hemoglobin, [7, 8] providing an estimate of what the patient’s expected DLCO should be given their hemoglobin level, equipment manufacturers have been slow to offer this accommodation in the testing software and the older practice of adjusting the patient’s measured DLCO to what it would be if their hemoglobin was normal is still quite common. Both methods are presented below. Both methods yield identical values when the measured values are compared with the predicted values and expressed as a percentage of the predicted value. Regardless of whether the measured or predicted values are adjusted, both adjusted and unadjusted values should be displayed on the final report, along with the measured hemoglobin (and date of hemoglobin determination).

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