A PFT with DLCO is a set of pulmonary function tests that measures airflow, lung volumes and how well gases transfer from air sacs into blood. Spirometry assesses breathing mechanics; DLCO measures diffusion capacity using a tiny carbon monoxide test breath. It helps diagnose and monitor COPD, interstitial lung disease and pulmonary vascular disease, guides treatment, and is noninvasive and typically completed within 20–30 minutes.

A high DLCO means the lungs transfer more carbon monoxide than expected, suggesting increased pulmonary capillary blood volume, higher hemoglobin, or enhanced alveolar-capillary conductance. Common causes include polycythemia, pulmonary hemorrhage, asthma, left-to-right cardiac shunts, and other conditions that raise pulmonary blood flow (for example, exercise or being supine). Clinical correlation and further testing are needed to identify the cause.

DLCO measures gas transfer across the alveolar–capillary membrane using carbon monoxide; it detects diffusion impairment from interstitial lung disease, emphysema, or pulmonary vascular disease and is affected by hemoglobin and pulmonary blood flow. Spirometry measures airflow and volumes (FEV1, FVC, FEV1/FVC), diagnoses obstructive versus restrictive patterns, and is effort‑dependent and used for bronchodilator testing.

DLCO is typically reduced in COPD, especially when emphysema predominates, because alveolar wall destruction and loss of pulmonary capillary bed impair gas diffusion. In chronic bronchitis–dominant COPD, DLCO may be normal or only mildly decreased. The degree of DLCO reduction correlates with emphysema severity, exercise limitation, and prognosis.

Lung diseases that affect DLCO include those that reduce it—emphysema/COPD, interstitial lung disease (pulmonary fibrosis), pulmonary vascular disease (pulmonary hypertension, chronic thromboembolic disease) and acute pulmonary embolism—as well as alveolar‑filling disorders (pulmonary edema). Diffuse alveolar hemorrhage can raise DLCO; asthma often shows normal or increased DLCO. Hematologic factors (anemia, polycythemia) also alter DLCO.

A pulmonary function test (PFT) measures how well your lungs work by assessing airflow, lung volumes and gas exchange. Typical components include spirometry, lung-volume measurements and the diffusion (DLCO) test. PFTs help diagnose and monitor asthma, COPD and restrictive lung disease and evaluate fitness for surgery. They are noninvasive: you breathe into a mouthpiece, sometimes before and after a bronchodilator; testing takes 15–60 minutes.