Single breath diffusing capacity for carbon monoxide: Effects of adjustment for inspired volume dead space, carbon dioxide, hemoglobin and carboxyhemoglobin

In order to assess the additive effects of taking into account dead space volume (V-D), carbon dioxide, hemoglobin (Hb) and carboxyhemoglobin on computation of single breath carbon monoxide diffusing capacity (D(L)COsb), we sequentially applied all the corrections recommended by the 1987 American Thoracic Society (ATS) document on D(L)COsb standardization. We used data from 739 men (333 nonsmokers and 406 current smokers) and 475 women (403 nonsmokers and 72 current smokers) who underwent measurement of D(L)COsb in the decade 1985-1994 at the Lung Function Laboratory of our institute. With respect to the unadjusted D(L)COsb value, significant small differences were found for all the corrected formulas, ranging from -0.18 to 1.48 ml/min/mm Hg in men and from -0.24 to 1.57 ml/min/mm Hg in women. After computing the percent change of D(L)COsb [(unadjusted-adjusted value) x 100/unadjusted value], we observed that the correction for V-D caused an underestimation of D(L)COsb of about 5.8% in men and 7.7% in women. However, when all the corrections were considered, these figures decreased to about 0.9% in males and 2.9% in females. Regarding specifically the correction for Hb, the adjusted value was slightly lower in men, while it was somewhat higher in women, with respect to the unadjusted D(L)COsb. In conclusion, the corrections suggested by ATS in the computation of D(L)COsb, when considered altogether, seem to account for a limited proportion of test variability in usual clinical conditions, especially in males