Pulmonary Function Tests: How to Interpret Spirometry and DLCO Results

When your doctor says you need a pulmonary function test, it’s not just another box to check. These tests tell you what’s really going on inside your lungs-especially when you’re short of breath, coughing constantly, or just feel like you can’t catch your air anymore. Two of the most important tests are spirometry and DLCO. They’re simple, non-invasive, and packed with information. But if you don’t know how to read them, you’re missing half the story.

What Spirometry Actually Measures

Spirometry is the first test most people get when lung problems are suspected. You breathe in as deep as you can, then blow out as hard and fast as you can into a tube connected to a machine. It sounds easy, but it’s not. You have to give it your all-no half-hearted attempts.

The machine records two key numbers: FEV1 (how much air you can force out in the first second) and FVC (how much you can blow out total). Then it calculates the ratio: FEV1 divided by FVC. That ratio is everything.

If your FEV1/FVC ratio is below 0.7, you likely have obstruction-think asthma, COPD, or emphysema. Your airways are narrowed, so air doesn’t flow out as quickly. But if the ratio is normal and your FVC is low? That’s restriction. Your lungs aren’t expanding fully. Could be scarring from past infections, obesity, or something like pulmonary fibrosis.

Here’s the catch: spirometry alone can’t tell you why your lungs aren’t expanding. Is it because your chest wall is stiff? Or because your lung tissue itself is damaged? That’s where DLCO comes in.

What DLCO Reveals That Spirometry Can’t

DLCO stands for diffusing capacity of the lung for carbon monoxide. Sounds complicated? It’s not. It measures how well oxygen moves from your lungs into your blood. That’s the whole point of having lungs-to get oxygen into your bloodstream.

For this test, you inhale a tiny, harmless mix of gases-carbon monoxide, helium, oxygen-and hold your breath for exactly 10 seconds. Then you exhale. The machine compares how much CO you breathed in versus how much came out. The difference tells you how much was absorbed by your blood.

Normal DLCO is between 75% and 140% of what’s predicted for your age, height, and sex. Below 75%? Something’s wrong with gas exchange. Above 140%? That’s unusual, but it can happen in conditions like polycythemia or early asthma attacks.

The real power of DLCO is in the details. If you have low FVC (restriction) but normal DLCO, your lungs are probably physically restricted-maybe from obesity or scoliosis. The tissue itself is fine. But if your DLCO is low too? That’s a red flag for lung damage-interstitial fibrosis, emphysema, or pulmonary hypertension. The tiny air sacs (alveoli) or blood vessels are failing.

Putting It Together: Real Patterns You’ll See

Here’s how these tests work together in real life:

Asthma: Spirometry may show obstruction (low FEV1/FVC), but DLCO is often normal-or even high during flare-ups. Why? Because your airways are inflamed, but your gas exchange is still working fine.
Emphysema: FEV1/FVC is low (obstruction), and DLCO is almost always low. The walls between air sacs are destroyed. Less surface area = less oxygen transfer.
Pulmonary fibrosis: Both FVC and DLCO are low. But DLCO drops first-sometimes months before spirometry shows anything. That’s why it’s so valuable for early detection.
Pulmonary hypertension: Spirometry might look normal. But DLCO is low, and the ratio of FVC to DLCO is often above 1.6. This tells doctors the problem is in the blood vessels, not the lung tissue.
Chronic pulmonary embolism: This one’s sneaky. Spirometry can be normal. But DLCO is low because clots block blood flow in the lungs. It’s one of the few conditions where DLCO is the main diagnostic clue.

Person inhaling gas mixture as oxygen transfers from lungs to blood

Why DLCO Is Often Misunderstood

Even among doctors, DLCO gets overlooked. Why? Because it’s not as straightforward as spirometry. Several things can throw it off:

Anemia: Low hemoglobin means less oxygen-carrying capacity. That lowers DLCO-even if your lungs are perfectly healthy. A 1 g/dL drop in hemoglobin can reduce DLCO by about 1%.
Smoking: Carbon monoxide from cigarettes sticks to hemoglobin, making it harder for the test to measure true uptake. Smokers often get falsely low DLCO readings.
Age and height: DLCO naturally declines with age. Taller people have higher values. Reference numbers must be adjusted.
Breath-hold timing: If you hold your breath for 8 seconds instead of 10, your DLCO will be wrong. That’s why proper technique matters.

That’s why every DLCO test should come with your hemoglobin level. Without it, you’re guessing.

When Doctors Order These Tests

You won’t get these tests unless there’s a reason. Common scenarios:

You’re short of breath and no one knows why.
You have a chronic cough or wheeze that doesn’t respond to inhalers.
You’re being evaluated for lung surgery.
You have a connective tissue disease like scleroderma or lupus-both can quietly damage your lungs.
You’re being monitored for pulmonary fibrosis. DLCO is tracked every 3-6 months to see if the disease is progressing.

In fact, if you’ve been diagnosed with pulmonary fibrosis, your DLCO is one of the most important numbers your doctor watches. If it drops below 35% of predicted, your risk of death over the next few years increases nearly threefold.

What If Your Results Don’t Match Your Symptoms?

Sometimes you feel awful, but your tests look normal. That’s frustrating. But it doesn’t mean nothing’s wrong.

If your spirometry and DLCO are both normal but you’re still struggling to breathe, the issue might be in your chest wall, diaphragm, or even your heart. Or it could be anxiety. But don’t assume that. Ask your doctor about bronchoprovocation testing (to check for asthma) or a six-minute walk test (to see how your oxygen drops with activity).

And if your DLCO is low but your spirometry is fine? That’s a red flag for early disease. Don’t wait. Follow up with a high-resolution CT scan. You might have early interstitial lung disease-something that’s treatable if caught early.

Comparison of three lung conditions with test values and key factors

What to Do After Your Test

Don’t just get your results and walk away. Ask:

What’s my FEV1/FVC ratio? Is it below 0.7?
Is my DLCO below 75%? If so, what could be causing it?
Was my hemoglobin checked? Can I see that number?
Do I need a CT scan or further testing?

Bring a friend. Take notes. Write down the numbers. These tests aren’t just for doctors-they’re for you too.

Future of Lung Testing

Artificial intelligence is starting to help interpret these tests. A 2023 study from Mayo Clinic showed AI could predict pulmonary hypertension from DLCO patterns with 88% accuracy. That’s not replacing doctors-it’s helping them see patterns faster.

And the tests themselves? They’re not going anywhere. Medicare still pays $85-$110 for DLCO. Hospitals still use them. And for good reason: they’re cheap, safe, and more informative than most blood tests.

The bottom line? Spirometry tells you if air is flowing. DLCO tells you if oxygen is getting through. Together, they give you the full picture. Ignore one, and you’re flying blind.

What does a low DLCO mean?

A low DLCO means your lungs aren’t transferring oxygen into your blood as well as they should. This can happen with emphysema, pulmonary fibrosis, pulmonary hypertension, chronic pulmonary embolism, or anemia. It’s not a diagnosis by itself-it’s a clue that points to damage in the tiny air sacs or blood vessels of the lungs.

Can you have normal spirometry but still have lung disease?

Yes. Many early lung diseases, like pulmonary fibrosis or chronic pulmonary embolism, show up first as a low DLCO-even when FEV1 and FVC are normal. That’s why DLCO is so important. It catches problems spirometry misses.

Why is hemoglobin important for DLCO?

Hemoglobin carries oxygen in your blood. If your hemoglobin is low (anemia), your DLCO will be artificially low-even if your lungs are healthy. Every DLCO result should be interpreted with your hemoglobin level. A drop of 1 g/dL in hemoglobin can lower DLCO by about 1%.

Is DLCO testing painful?

No. It’s completely painless. You just breathe in a harmless gas mixture, hold your breath for 10 seconds, then breathe out. Some people feel lightheaded, but it’s rare. The test takes less than 5 minutes.

How often should DLCO be repeated?

For people with stable lung disease, once a year is typical. For progressive conditions like pulmonary fibrosis, it’s often checked every 3-6 months to track how fast the disease is advancing. If you’re on a new treatment, your doctor may check it sooner to see if it’s working.

Can smoking affect DLCO results?

Yes. Smoking raises carboxyhemoglobin levels in your blood, which interferes with the test. This can make your DLCO look lower than it really is. If you smoke, your doctor should know-because they may need to adjust the results or ask you to stop smoking for a few days before testing.

What’s the difference between restriction and obstruction?

Obstruction means your airways are narrowed-air flows out slowly. Think asthma or COPD. Restriction means your lungs can’t expand fully-like with scarring or obesity. Spirometry shows both, but DLCO helps tell you why: is it the lung tissue itself (low DLCO), or just the chest wall (normal DLCO)?

Can DLCO be normal in someone with emphysema?

Rarely. Emphysema destroys the air sacs, so DLCO is almost always low. But in very early stages, or if the damage is patchy, DLCO might still be in the normal range. That’s why a normal DLCO doesn’t rule out emphysema-CT scans are needed for confirmation.

Next Steps If You’re Concerned

If your test results are abnormal or unclear, don’t wait. Ask for a referral to a pulmonologist. Bring your test printouts. Ask what the numbers mean for your daily life. Ask if you need a CT scan, a walk test, or a sleep study.

Lung disease doesn’t always scream. Sometimes it whispers. Spirometry and DLCO are the tools that help you hear it before it’s too late.

15 Comments

Steve Sullivan
December 9, 2025 AT 19:19

bro this is the most useful thing i've read all week 🙏 i literally just got my DLCO results and was like wtf is this number but now i get it. thanks for breaking it down like i'm 5
Maria Elisha
December 11, 2025 AT 18:56

I didn't even know DLCO stood for anything. I just thought it was a weird lab code.
Tejas Bubane
December 13, 2025 AT 08:46

This post is 90% common sense and 10% overcomplicated jargon. If you're a doctor you already know this. If you're not, you're still lost. Why not just say 'if your lungs suck, get a CT' and be done with it?
Lisa Whitesel
December 14, 2025 AT 01:06

People don't understand that DLCO isn't a diagnostic tool. It's a clue. But no one wants to hear that. They want a label. That's why medicine is broken.
Andrea DeWinter
December 14, 2025 AT 22:23

I work with patients who have pulmonary fibrosis every day. DLCO is the first thing we watch. If it drops 10% in 6 months, we change the game plan. It's not just a number-it's a warning bell. And yes, hemoglobin matters. I've seen people panic over a low DLCO only to find out they were anemic. Always check the CBC first.
Sabrina Thurn
December 15, 2025 AT 01:16

The FEV1/FVC ratio being <0.7 is the gold standard for obstruction, but I always caution against over-reliance on cutoffs. The lower limit of normal (LLN) is more accurate, especially in older adults or those with extreme height. Many guidelines still use 0.7 because it's simple, but it overdiagnoses COPD in the elderly. Context is everything.
Angela R. Cartes
December 15, 2025 AT 06:40

I love how this post makes me feel smart 😌 I mean, I didn't know DLCO was about oxygen transfer, but now I can impress my doctor with my 'knowledge'. Also, I'm definitely asking for my hemoglobin next time. I'm not some lab rat.
Andrea Beilstein
December 16, 2025 AT 07:51

There's a philosophical dimension here. The body speaks in numbers, but we demand narratives. We want a story: 'You have asthma.' 'You have fibrosis.' But the truth is messier. DLCO doesn't lie, but it doesn't speak in sentences either. It whispers in millimeters of mercury and percentages. Are we listening-or just collecting data to validate our fears?
Larry Lieberman
December 17, 2025 AT 19:52

Wait so if I have normal spirometry but low DLCO, does that mean I could have a blood clot in my lungs and not know it? 🤯 that's wild. I thought only CT scans caught that. This changes everything.
precious amzy
December 18, 2025 AT 08:30

One must question the epistemological foundations of pulmonary diagnostics. The reliance on population-based percentiles assumes homogeneity of lung architecture, which is a fallacy. DLCO, as a proxy for alveolar-capillary membrane efficiency, is inherently confounded by systemic variables-hemoglobin, cardiac output, even altitude. To treat this metric as a standalone diagnostic entity is to mistake correlation for causation. The reductionist paradigm of modern pulmonology is, frankly, intellectually bankrupt.
Ajit Kumar Singh
December 19, 2025 AT 06:37

I am from India and we have no access to these tests in rural areas. But I tell my patients: if you can't walk up one flight without gasping, you have lung disease. No machine needed. Why pay $100 for a test when a stair test tells you everything?
iswarya bala
December 20, 2025 AT 07:09

this is so helpful!! i had no idea smoking messes with dlco 😳 i thought it was just for cancer risk. i'm gonna quit for a week before my next test. thank you thank you thank you
Simran Chettiar
December 21, 2025 AT 19:04

It is imperative to recognize that the interpretation of pulmonary function tests is not merely a mechanical exercise in numerical comparison, but rather a hermeneutic process requiring synthesis of clinical context, patient history, and physiological principles. The reduction of DLCO to a single percentage value, divorced from the patient's activity tolerance, comorbidities, and smoking status, constitutes a fundamental epistemic error in contemporary medical practice.
om guru
December 23, 2025 AT 17:33

Good information. Keep it simple. Patient should know: if FEV1/FVC is low, airways are tight. If DLCO is low, oxygen is not getting in. If both are low, lungs are damaged. If only DLCO is low, check for clots or anemia. If normal but you still can't breathe, check heart or anxiety. That's all you need.
Brianna Black
December 25, 2025 AT 13:13

I cried reading this. My mom had pulmonary fibrosis. She was told her spirometry was 'mildly abnormal' for five years. Then one day, her DLCO dropped from 68% to 41% in six months. That's when they finally did the CT. That's when we knew. This isn't just science-it's survival. Thank you for writing this. I'm sharing it with every person I know who's ever been told 'it's just aging'.