Noninvasive measurement of carboxyhemoglobin levels for adjustment of diffusion capacity measured during pulmonary...

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.

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.

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.

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.