Retrospective study of pulmonary function tests in patients presenting with isolated reduction in single-breath diffusion capacity: implications for the diagnosis of combined obstructive and restrictive lung disease.

Objective: To examine the frequency and spectrum of diseases associated with isolated reduction in the diffusing capacity of lung for carbon monoxide (D(Lco)).

Patients And Methods: We retrospectively identified all potentially dyspneic patients who had pulmonary function tests (PFTs) performed at the Mayo Clinic in Jacksonville, Fla, between January 1, 1990, and June 30, 2000, that showed reduced D(Lco) (< 70% of predicted), normal lung volumes (total lung capacity and residual volume > 80% and < 120% of predicted, respectively), and airflow variables (forced expiratory volume in 1 second and forced vital capacity values > 80% of predicted and forced expiratory volume in 1 second/forced vital capacity ratio > 70% of predicted). Only patients who had also undergone chest computed tomography (CT) and echocardiography within 1 month of PFTs were studied.

Results: Of the 38,095 patients who underwent PFTs during the study period, 179 (0.47%; 95% confidence interval [CI], 0.40%-0.54%) had isolated D(Lco) abnormalities. The 27 patients (15.1%; 95% CI, 10.2%-21.2%) who had also undergone chest CT and echocardiography within 1 month of PFTs form the study cohort reported herein. Their mean D(Lco) was 50% +/- 15% (95% CI, 45%-56%) with average normal pulse oxygen saturation at rest and mild hypoxemia with activity. Thirteen of the 27 patients (48%; 95% CI, 28.7%-68.1%) had underlying emphysema evident on CT. Eleven of these 13 patients had emphysema associated with a restrictive lung process. The 14 patients without emphysema had interstitial lung disease, pulmonary vascular disease, and other isolated findings. Six patients with combined emphysema and idiopathic pulmonary fibrosis accounted for the largest percentage (22%) of patients with Isolated D(Lco) reduction. The mean +/- SD smoking history of the 27 patients in the study cohort was 36 +/- 33 pack-years (range, 0-116 pack-years).

Conclusion: Dyspneic patients with respiratory symptoms and normal lung volumes and airflows associated with Isolated reduction in D(Lco) should be evaluated for underlying diseases such as emphysema, with or without a concomitant restrictive process, and pulmonary vascular disease.