DLCO: What It Is, Why It Matters, and How It Guides Lung Health Decisions

When your doctor orders a DLCO, the diffusing capacity of the lungs for carbon monoxide, a test that measures how well oxygen moves from your lungs into your bloodstream. Also known as DLCO, it’s one of the most telling parts of a pulmonary function test—especially when you’re dealing with shortness of breath that doesn’t make sense on a chest X-ray. Unlike simple breathing tests that just check how much air you can blow out, DLCO looks at the actual exchange of gas at the tiny air sacs in your lungs. If those sacs are damaged—by smoking, scarring, or disease—your body can’t pull in enough oxygen, even if your lungs feel full.

This test is critical for conditions like pulmonary fibrosis, a disease where lung tissue becomes stiff and scarred, making gas exchange harder, or COPD, a group of lung diseases including emphysema that destroy the air sacs and capillaries. It also helps track how well treatments like oxygen therapy or lung transplants are working. People on medications that can harm the lungs, like some chemotherapy drugs, often get DLCO tests to catch damage early. Even heart failure can show up as a low DLCO because poor blood flow means less oxygen gets picked up, even if the lungs themselves are fine.

What you’ll find in the posts below isn’t theory—it’s real-world guidance. You’ll see how DLCO results are interpreted alongside other tests, why some people with normal breathing numbers still have dangerously low DLCO, and how doctors use this number to decide between medications, oxygen, or even surgery. There’s also advice on preparing for the test so you get accurate results, and what to ask your doctor if your numbers drop over time. This isn’t just about numbers on a page—it’s about understanding whether your lungs are still doing their job, and what to do next if they’re not.