When your doctor orders pulmonary function tests, it’s not just another checkbox. These tests-especially spirometry and DLCO-can reveal what’s really going on inside your lungs, even when you feel fine. Many people assume normal breathing means healthy lungs. But that’s not always true. You could be struggling with early-stage lung disease and still take full breaths without wheezing or coughing. That’s why these tests matter.
What Spirometry Actually Measures
Spirometry is the most common lung test you’ll ever take. You breathe in as deeply as you can, then blow out as hard and fast as you can into a tube connected to a machine. It sounds simple. But the numbers it produces tell a detailed story.
The two most important numbers are FEV1 and FVC. FEV1 is how much air you can force out in the first second. FVC is the total amount you can blow out in one full breath. The ratio between them-FEV1/FVC-is the real key. If that ratio drops below 0.7, it usually means your airways are narrowed. That’s the hallmark of obstructive diseases like asthma or COPD.
But here’s where people get confused: a low FVC doesn’t always mean the same thing. If your FEV1/FVC ratio is normal but your FVC is low, that suggests restriction-not obstruction. Your lungs might be physically smaller, or something outside the lungs is limiting their expansion. That could be scarring from past infection, obesity, or even severe spinal curvature.
Normal values are based on your age, height, sex, and ethnicity. A 70-year-old woman who’s 5’2” won’t have the same numbers as a 30-year-old man who’s 6’1”. Labs use reference equations from the Global Lung Function Initiative to adjust for this. Anything above 80% of predicted is generally considered normal.
What DLCO Tells You That Spirometry Can’t
DLCO-diffusing capacity of the lung for carbon monoxide-is where things get more complex. Most patients haven’t heard of it. Even some doctors don’t order it routinely. But when you have unexplained shortness of breath and normal spirometry, DLCO is often the missing piece.
This test measures how well oxygen moves from your lungs into your bloodstream. You inhale a tiny, harmless amount of carbon monoxide mixed with helium and oxygen. You hold your breath for exactly 10 seconds, then exhale. The machine measures how much CO was absorbed. That absorption rate is your DLCO.
A normal DLCO is between 75% and 140% of predicted. Below 75% means your lungs aren’t transferring oxygen efficiently. Above 140% is unusual but can happen in conditions like polycythemia or early asthma.
Why does this matter? Because two people can have the same low FVC on spirometry but very different DLCO results. One might have pulmonary fibrosis-the lung tissue is scarred, so oxygen can’t cross into the blood. Their DLCO will be very low. The other might have obesity or scoliosis-the lungs are compressed, but the tissue itself is fine. Their DLCO will be normal or even high.
This distinction changes everything. One needs a lung biopsy. The other needs weight loss or physical therapy.
When DLCO Is Low: The Real Clues
A low DLCO doesn’t point to one disease. It points to a problem in gas exchange. That means something’s wrong with the alveoli, the capillaries, or the blood.
Here’s what it usually means:
- Interstitial lung disease (ILD): Scarring from sarcoidosis, asbestosis, or idiopathic pulmonary fibrosis. DLCO drops early-sometimes before spirometry shows anything. In advanced IPF, DLCO can fall below 20% of predicted, even when FVC is still at 60%.
- Emphysema: The walls between air sacs break down. Less surface area means less oxygen transfer. DLCO is often low, even if FEV1/FVC is borderline.
- Pulmonary hypertension: Blood vessels in the lungs narrow. Blood flow slows. DLCO drops because less blood reaches the alveoli. The FVC/DLCO ratio is often above 1.6 in these cases.
- Chronic pulmonary embolism: Blood clots block vessels. DLCO is frequently low, with sensitivity up to 85%. Many patients are misdiagnosed with asthma until DLCO reveals the truth.
- Connective tissue diseases: Lupus, scleroderma, and rheumatoid arthritis can silently damage lung tissue. DLCO is often the first sign.
And here’s the kicker: you can have normal spirometry and still have a low DLCO. That’s why skipping DLCO is like checking the engine light but not looking under the hood. If you’re short of breath and your spirometry is normal, ask for DLCO.
When DLCO Is High: Less Common, But Important
High DLCO is rarer, but it’s not always a good thing.
- Asthma: During an attack, blood flow increases in the lungs. DLCO can rise to 120-140% of predicted. That’s why some patients with asthma have normal spirometry but high DLCO.
- Polycythemia: Too many red blood cells mean more hemoglobin to grab oxygen. DLCO goes up.
- Pulmonary hemorrhage: Blood leaks into the alveoli. Hemoglobin from the blood absorbs CO, making DLCO appear falsely high.
- Left-to-right heart shunts: Blood bypasses the lungs, then re-enters circulation. The body compensates by increasing blood flow to the lungs, raising DLCO.
These aren’t just academic distinctions. They change treatment. A patient with high DLCO and asthma needs different meds than someone with high DLCO due to polycythemia.
Why Hemoglobin Matters More Than You Think
DLCO isn’t just about lungs. It’s about blood too. Hemoglobin carries the CO you inhale. If you’re anemic, your DLCO will be lower-even if your lungs are perfectly healthy.
For every 1 g/dL drop in hemoglobin, DLCO falls by about 1%. So if your hemoglobin is 10 instead of 14, your DLCO could be 4% lower. That’s enough to push a normal result into the “low” range.
That’s why labs must measure hemoglobin before the test. If they don’t, you could get a false diagnosis of lung disease. Many clinics still skip this step. Always ask: “Was my hemoglobin checked?”
Smoking also affects DLCO. Carbon monoxide from cigarettes binds to hemoglobin, leaving less room for the test gas. That can falsely lower DLCO by 5-10%. If you smoke, you should avoid cigarettes for at least 24 hours before the test.
How the Tests Work Together
Doctors don’t interpret these tests alone. They use them like a puzzle.
Here’s how the logic flows:
- If FEV1/FVC is below 0.7 → Obstruction. Think asthma, COPD. DLCO helps tell if emphysema is involved.
- If FEV1/FVC is normal but FVC is low → Restriction. Now check DLCO.
- If DLCO is low → Intraparenchymal problem (lung tissue damage). Think fibrosis, emphysema, pulmonary hypertension.
- If DLCO is normal → Extraparenchymal problem (outside the lung). Think obesity, scoliosis, neuromuscular disease.
- If FEV1 and FVC are normal, but you’re short of breath → Check DLCO. It could be early ILD, pulmonary embolism, or heart issues.
The FVC/DLCO ratio is another clue. If it’s above 1.6, pulmonary hypertension is likely. If it’s below 1.3, you’re more likely dealing with parenchymal lung disease.
Common Mistakes in Interpretation
Even experienced doctors misread these tests. Here are the top three errors:
- Ignoring hemoglobin: Calling a low DLCO “lung disease” when it’s just anemia.
- Assuming normal spirometry = normal lungs: Missing early fibrosis or pulmonary embolism because they don’t affect airflow yet.
- Using old reference values: Some labs still use outdated equations from the 1980s. The Global Lung Function Initiative standards are now the gold standard.
Also, don’t forget technical factors. If you can’t hold your breath for 10 seconds-common in older adults or people with dementia-the DLCO result is unreliable. Some patients need multiple attempts. A bad test isn’t a bad result-it’s a bad test.
What Happens Next?
If your DLCO is low and spirometry is normal, your doctor might order a high-resolution CT scan. That’s the next step to see if there’s scarring or nodules.
If you have COPD and a low DLCO, you might be referred to a specialist for emphysema evaluation. In some cases, lung volume reduction surgery or targeted meds become options.
If you have fibrosis and DLCO is below 35%, your prognosis changes. Studies show survival drops significantly. That’s when treatments like pirfenidone or nintedanib are considered.
And if you’re preparing for surgery-especially lung removal-DLCO helps predict if you’ll tolerate it. If your DLCO is below 40%, your risk of complications rises.
Why These Tests Won’t Disappear
These aren’t old-school tools fading into obsolescence. They’re evolving. In 2023, Mayo Clinic published a study using AI to predict pulmonary hypertension from DLCO patterns with 88% accuracy. That’s not science fiction-it’s happening now.
Insurance still covers them. CPT codes 94010 (spirometry) and 94720 (DLCO) are reimbursed at $45-$65 and $85-$110 respectively. Usage has grown 4.2% annually since 2019. Pulmonary fibrosis centers rely on DLCO to track disease progression in over 200,000 patients.
They’re cheap, non-invasive, and give answers no X-ray or MRI can. You don’t need a needle, radiation, or contrast. Just breath.
So if you’ve been told your lungs are fine but you still can’t catch your breath-ask for DLCO. It might be the test that finally explains why.
