Study objective: Single-breath diffusing capacity of the lung for carbon monoxide (Dlco) is used as a pulmonary function test (PFT) to assess gas transfer in the lungs. The implications of a low Dlco are well-recognized, but the clinical significance of a high Dlco is not clear. The aim of this study was to identify the clinical correlates of a high Dlco.
Patients and methods: We identified 245 patients with a high Dlco (ie, > 140% predicted) and a matched group of 245 patients with normal Dlco (ie, 85 to 115% predicted), who were selected from a laboratory database of 45,000 patients tested between January 1997 and December 1999. We compared the demographic features, clinical diagnoses, and PFT data between the two groups.
Settings: Large multispecialty group practice.
Results: The patients in the high Dlco group were heavier (mean [± SD] weight, 96.0 ± 22.9 vs 85.0 ± 21.3 kg, respectively; p < 0.001), had a higher mean body mass index (32.9 ± 7.4 vs 29.4 ± 6.4 kg/m2, respectively; p < 0.001), larger body surface area (p < 0.001), and larger mean total lung capacity (p = 0.007) and alveolar volume (p < 0.001). The clinical diagnoses of obesity (p < 0.001) and asthma (p < 0.001) were more common among patients with high Dlco values. The majority of patients (62%) with a high Dlco had a diagnosis of obesity, asthma, or both. Polycythemia, hemoptysis, and left-to-right shunt were uncommon.
Conclusion: A high Dlco on a PFT is most frequently associated with large lung volumes, obesity, and asthma. Other conditions are much less common. A clinical condition, which typically reduces Dlco, may deceptively normalize Dlco in such patients.