Which factors affect diffusing capacity of lung for carbon monoxide (DLCO) values in pulmonary function testing?

Updated: May 14, 2020
  • Author: Kevin McCarthy, RPFT; Chief Editor: Nader Kamangar, MD, FACP, FCCP, FCCM  more...
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Answer

Answer

Other factors have been shown to impact the measured DLCO, such as elevated blood carboxyhemoglobin (COHb) and barometric pressure. The impact of an elevated carboxyhemoglobin is twofold: (1) it reduces the alveolar-capillary pressure gradient for CO and (2) acts as a virtual anemia by holding onto sites on the hemoglobin molecule that could be used for binding CO (or oxygen). The net effect is a 2% decrease in DLCO for each 1% increase in COHb. RGA systems can measure the CO in the patient’s exhaled breath just prior to inhalation of the DLCO test gas and compensate for elevated CO by subtracting the estimated CO back-pressure from both the initial and final alveolar carbon monoxide partial pressures. This compensates for the reduced alveolar-capillary pressure gradient but does not compensate for the anemia effect. The 2017 DLCO standards paper shows a formula that also adjusts for the anemia effect, but this is not currently in use on most PFT systems.


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