Adjustment of DL(co) for hemoglobin concentration

The equation proposed by Cotes and coworkers is currently considered as the most acceptable to correct carbon monoxide diffusing capacity (DL(CO)) for hemoglobin concentration [Hb] by both the American Thoracic Society (ATS) and the European Respiratory Society (ERS) guidelines for standardization of DL(CO). In a previous study on 24 anemic patients undergoing bone marrow transplantation (1), we found that DL(CO) is underestimated using the equation of Cotes and coworkers. To further explore this finding, 28 anemic patients ([Hb] = 8.2 +/- 1.0(SD)g/dl) with chronic renal failure were prospectively studied during the recovery period of anemia (5.4 +/- 3.5 mo). In all 28 subjects, the slope Delta DL(CO)/Delta[Hb] computed as ratio of overall change in DL(CO) to overall change in [Hb] throughout the study period was 1.40 +/- 0.72 mi CO/min/mm Hg/g/dl. The individual relationship between measured DL(CO) and [Hb] closely fitted a simple linear regression. The resulting equations for adjustment of DL(CO) (DL(CO)adj) to a standard [Hb] of 14.6 g/dl for men and 13.4 g/dl for women are: DL(CO)adj (men) = DL(CO)obs + 1.40(14.6 - [Hb]) DL(CO)adj (women) = DL(CO)obs + 1.40(13.4 - [Hb]) The present adjustment function for DL(CO) is linear and independent of the observed DL(CO) values, whereas the formulas previously proposed are curvilinear, DL(CO) correction varying with the measured DL(CO) values. For a measured DLCO of 15 mi CO/min/mm Hg and [Hb] ranging from 7 to 12 g/dl, the present DL(CO) adjustment is higher (by 2.7 mi CO/min/mm Hg, on average) than that proposed by Cotes and coworkers. This difference appears to be relevant for a precise interpretation of DL(CO) in patients with normocytic anemia in different clinical conditions.