Pulmonary Function Tests: How to Interpret Spirometry and DLCO Results

When your doctor orders pulmonary function tests, they’re not just checking if your lungs are working. They’re trying to figure out why you’re short of breath, coughing, or feeling tired after climbing stairs. Two of the most important tests in this process are spirometry and DLCO. Together, they give a complete picture of how well your lungs move air and transfer oxygen into your blood. But interpreting these numbers isn’t as simple as reading a thermometer. You need to understand what each number means, how they relate to each other, and what they reveal about your lung health.

What Spirometry Tells You About Airflow

Spirometry is the most common lung test you’ll ever have. It’s quick, non-invasive, and done in almost every clinic. You take a deep breath and blow out as hard and fast as you can into a tube connected to a machine. The machine measures two key numbers: FEV1 (how much air you can force out in the first second) and FVC (the total amount of air you can blow out after a full inhale).

The real magic is in the ratio: FEV1 divided by FVC. If this ratio is below 0.7, it usually means you have airflow obstruction - like in asthma or COPD. Your airways are narrowed, so you can’t empty your lungs quickly. A normal ratio (above 0.7) with a low FVC suggests restriction - your lungs can’t expand fully. But here’s the catch: sometimes, severe air trapping from COPD can make FVC look low even though your lungs aren’t truly restricted. That’s where DLCO comes in.

Why DLCO Is the Hidden Key

DLCO stands for diffusing capacity of the lung for carbon monoxide. It sounds complicated, but it’s really measuring one thing: how well oxygen moves from your lungs into your bloodstream. The test uses a harmless gas mixture with a tiny bit of carbon monoxide. You inhale it, hold your breath for 10 seconds, then exhale. The machine compares how much CO you breathed in versus how much came out. Less CO in the exhaled air means more got absorbed - which is good. But if your lungs are damaged, less CO gets absorbed, and the number drops.

DLCO is normal in most cases of asthma or COPD where the problem is just airway narrowing. But if your DLCO is low, something deeper is wrong. It points to damage in the tiny air sacs (alveoli) or the blood vessels around them. This is why DLCO is critical for spotting early interstitial lung disease, pulmonary fibrosis, or even pulmonary hypertension before spirometry shows any changes.

Putting Spirometry and DLCO Together

Here’s how the pieces fit:

• Low FEV1/FVC + Normal DLCO → Classic obstructive disease like asthma or COPD. The problem is narrowed airways, not damaged lung tissue.
• Low FVC + Low DLCO → Restrictive disease with lung damage. Think pulmonary fibrosis, sarcoidosis, or asbestosis. The lung tissue is stiff and scarred, so it can’t expand or transfer oxygen well.
• Low FVC + Normal DLCO → Restriction without lung damage. This could be from obesity, scoliosis, or weak breathing muscles. Your lungs are physically restricted, but the gas exchange is still fine.
• Normal spirometry + Low DLCO → This is the red flag most doctors miss. It could mean early interstitial lung disease, pulmonary embolism, or even early pulmonary hypertension. Even if you’re not wheezing or coughing, your lungs aren’t delivering oxygen properly.

One of the most telling patterns is the FVC/DLCO ratio. If it’s higher than 1.6, pulmonary hypertension is likely. This happens because the blood vessels in the lungs are thickened or blocked, making it harder for blood to flow through. The lungs compensate by increasing pressure, and DLCO drops while FVC stays normal or only slightly reduced.

When DLCO Is High - And What It Means

Most people assume low DLCO is the only concern. But sometimes, DLCO is higher than normal - above 140% of predicted. This can happen in:

• Asthma attacks (especially when severe), where increased blood flow to the lungs boosts gas transfer
• Pulmonary hemorrhage (bleeding into the lungs), where extra hemoglobin from blood cells grabs more CO
• Polycythemia (too many red blood cells), which increases the oxygen-carrying capacity
• Left-to-right heart shunts, where oxygen-rich blood recirculates through the lungs

These aren’t common, but they’re important to recognize. A high DLCO can mask an underlying problem - like a heart defect or a bleeding disorder - that’s being overlooked because the patient isn’t wheezing or coughing.

What Can Mess Up Your Results

DLCO is sensitive to things you might not think matter:

• Hemoglobin levels - Every 1 g/dL drop in hemoglobin lowers DLCO by about 1%. If you’re anemic, your DLCO will look worse than it really is.
• Smoking - Even if you quit a week ago, carbon monoxide in your blood from smoking can falsely lower DLCO by 5-10%.
• Breath-hold time - If you don’t hold your breath for exactly 10 seconds, the test is invalid. Many older or breathless patients struggle with this.
• Altitude - If you live at high elevation, your DLCO will naturally be higher. Labs use location-adjusted norms, but not all clinics do this correctly.

That’s why every good lab checks your hemoglobin before the test. If they don’t, the result is unreliable. And if you smoked the day before, tell your technician - they might reschedule.

Why Your Doctor Might Order DLCO Even If Spirometry Is Normal

If you’re short of breath but your spirometry looks perfect, it’s easy for doctors to say, “Your lungs are fine.” But that’s wrong. Many serious conditions show up only on DLCO:

• Early pulmonary fibrosis - DLCO drops 12-18 months before spirometry changes
• Chronic pulmonary embolism - DLCO is low in 85% of cases, even with normal breathing tests
• Connective tissue diseases (like scleroderma or lupus) - Lung involvement often shows first as a low DLCO
• Early emphysema - Especially in smokers with normal FEV1, DLCO can be the first warning sign

Dr. Neil MacIntyre from Duke University says DLCO is the least understood test in pulmonology - even among specialists. But if you have unexplained shortness of breath and normal spirometry, DLCO is the next step. Skipping it means missing early, treatable disease.

What Happens Next After the Test

Once you get your results, your doctor will look at the full picture:

• Is the pattern obstructive, restrictive, or mixed?
• Is DLCO in line with your FVC?
• Are there other symptoms - like joint pain, swelling, or coughing up blood?
• Do you have a history of smoking, asbestos exposure, or autoimmune disease?

If DLCO is low and the cause isn’t clear, you might need a high-resolution CT scan of your lungs. If pulmonary hypertension is suspected, an echocardiogram will follow. In cases of suspected interstitial lung disease, you may be referred to a specialist who manages these complex conditions.

DLCO is also used to track disease over time. In pulmonary fibrosis, a drop of more than 10% in DLCO over a year signals worsening disease and may trigger a change in treatment. The American Thoracic Society now uses DLCO below 35% of predicted as a marker of advanced disease - a key factor in deciding who needs lung transplant evaluation.

What to Ask Your Doctor

Don’t just accept the numbers. Ask:

• “Is my DLCO corrected for my hemoglobin level?”
• “Is this result consistent with my symptoms?”
• “Could this be something like pulmonary embolism or early fibrosis?”
• “Should I get a CT scan or see a lung specialist?”

If your doctor doesn’t mention DLCO’s role or can’t explain why it’s low, ask for a referral to a pulmonologist. These tests aren’t just routine - they’re your best shot at catching serious lung disease early.

Future of Lung Testing

New tools are emerging. A 2023 Mayo Clinic study showed AI algorithms can predict pulmonary hypertension from DLCO patterns with 88% accuracy. This means in the future, a simple DLCO test might flag high-risk patients before they even feel symptoms. But for now, the human interpretation of spirometry and DLCO - combined with clinical context - still beats any machine.

These tests are not just numbers on a page. They’re clues. And when you know how to read them, you can spot problems long before they become emergencies.

Understanding spirometry and DLCO isn’t just for doctors. If you’re living with a lung condition, knowing what these numbers mean helps you ask better questions, track your progress, and recognize when something’s off before it becomes serious.