Comparison of different measures of diffusing capacity for carbon monoxide (DLCO) in systemic sclerosis

In systemic sclerosis (SSc), impaired diffusing capacity for carbon monoxide (DLCO) can indicate interstitial lung disease (ILD), pulmonary hypertension (PH), and/or other disease manifestations, including anemia. We undertook this study to compare the various measures of DLCO in the setting of a complex disease like SSc. We analyzed the pulmonary function tests of a cohort of SSc subjects, as a whole and among subjects with isolated PH and ILD separately. Associations were assessed using Spearman correlation coefficients, Student's t tests, and F tests by one-way ANOVA. P values <0.05 were considered statistically significant. This study included 225 subjects (mean age, 57 years; 88 % women; mean disease duration, 9.6 years; 32 % with diffuse disease, 44 % with ILD, and 17 % with PH). Mean percent predicted DLCO values were 75 % for DLCOsb and 83 % for DLCOrb. Adjustment for alveolar volume (VA) resulted in near normalization of both DLCOsb/VAsb (91 %) and DLCOrb/VArb (91 %). Subjects with ILD had significantly lower DLCOsb but not DLCOsb/VAsb, whereas those with PH had significantly lower DLCOsb and DLCOsb/VAsb. Among the various measures of DLCO, DLCOsb had the strongest and most consistent associations with clinical outcomes of interest. Adjusting for alveolar volume dampened the associations except with PH, with which DLCOsb/VAsb was more strongly associated than DLCOsb. Low DLCOsb is the most sensitive measure to detect abnormalities in gas exchange in SSc but reflects both parenchymal lung disease and pulmonary vascular disease. Low DLCOsb/VAsb is more specific for pulmonary vascular disease and should be the preferred measure of gas exchange in SSc.