Noninvasive Measurement of Carboxyhemoglobin Levels for Adjustment of Diffusion Capacity Measured During Pulmonary Function Testing

BACKGROUND: The diffusing capacity of the lungs for carbon monoxide (D_LCO) is commonly measured during pulmonary function testing (PFT). Although adjustment of the measured D_LCO for an elevated baseline carboxyhemoglobin level is recommended, carboxyhemoglobin is not routinely measured, which may reduce the accuracy of D_LCO measurements. We sought to assess the utility of routine carboxyhemoglobin measurement and subsequent D_LCO correction in patients referred for PFT.

METHODS: We retrospectively reviewed 100 consecutive PFT results, including D_LCO assessment. We used a pulse CO-oximeter (recently approved by the Food and Drug Administration) to noninvasively measure baseline carboxyhemoglobin (S_pCO). We used simple descriptive statistics to compare the S_pCO values. In subjects with elevated S_pCO (> 2%) we adjusted the percent-of-predicted D_LCO. Interpretation of D_LCO was categorized according to the American Thoracic Society classification scheme for respiratory impairment.

RESULTS: The self-reported smokers had higher average S_pCO than did self-reported nonsmokers (1.6% vs 3.5%, p < 0.001), although 14% of nonsmokers had an elevated S_pCO and 26% of smokers had normal S_pCO. When the D_LCO was corrected for elevated S_pCO, 2 patients moved from a category of moderate impairment to mild impairment. Both were smokers.

CONCLUSIONS: The noninvasive measurement of carboxyhemoglobin is easy to perform during PFT. When precise measurement of D_LCO is important, noninvasive measurement of carboxyhemoglobin may be of value. If routine S_pCO measurement is considered, the highest yield is among current smokers.