Single Breath Diffusing Capacity for Carbon Monoxide: Effects of Adjustment for Inspired Volume Dead Space, Carbon Dioxide, Hemoglobin and Carboxyhemoglobin

Abstract

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_LCOsb), we sequentially applied all the corrections recommended by the 1987 American Thoracic Society (ATS) document on D_LCOsb 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_LCOsb in the decade 1985–1994 at the Lung Function Laboratory of our institute. With respect to the unadjusted D_LCOsb 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_LCOsb [(unadjusted – adjusted value) × 100/unadjusted value], we observed that the correction for V_D caused an underestimation of D_LCOsb 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_LCOsb. In conclusion, the corrections suggested by ATS in the computation of D_LCOsb, when considered altogether, seem to account for a limited proportion of test variability in usual clinical conditions, especially in males.