When your doctor orders pulmonary function tests, itâs not just another checkbox. These tests-especially spirometry and DLCO-can reveal whatâs really going on inside your lungs, even when you feel fine. Many people assume normal breathing means healthy lungs. But thatâs not always true. You could be struggling with early-stage lung disease and still take full breaths without wheezing or coughing. Thatâs why these tests matter.
What Spirometry Actually Measures
Spirometry is the most common lung test youâll 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. Thatâs the hallmark of obstructive diseases like asthma or COPD.
But hereâs where people get confused: a low FVC doesnât 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 whoâs 5â2â wonât have the same numbers as a 30-year-old man whoâs 6â1â. 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 Canât
DLCO-diffusing capacity of the lung for carbon monoxide-is where things get more complex. Most patients havenât heard of it. Even some doctors donât 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 arenât 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 canât 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 doesnât point to one disease. It points to a problem in gas exchange. That means somethingâs wrong with the alveoli, the capillaries, or the blood.
Hereâs 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 hereâs the kicker: you can have normal spirometry and still have a low DLCO. Thatâs why skipping DLCO is like checking the engine light but not looking under the hood. If youâre short of breath and your spirometry is normal, ask for DLCO.
When DLCO Is High: Less Common, But Important
High DLCO is rarer, but itâs not always a good thing.
- Asthma: During an attack, blood flow increases in the lungs. DLCO can rise to 120-140% of predicted. Thatâs 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 arenât 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 isnât just about lungs. Itâs about blood too. Hemoglobin carries the CO you inhale. If youâre 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. Thatâs enough to push a normal result into the âlowâ range.
Thatâs why labs must measure hemoglobin before the test. If they donât, you could get a false diagnosis of lung disease. Many clinics still skip this step. Always ask: âWas my hemoglobin checked?â
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 donât interpret these tests alone. They use them like a puzzle.
Hereâs how the logic flows:
- If FEV1/FVC is below 0.7 â Obstruction. Think asthma, COPD. DLCO helps tell if emphysema is involved.
- If FEV1/FVC is normal but FVC is low â Restriction. Now check DLCO.
- If DLCO is low â Intraparenchymal problem (lung tissue damage). Think fibrosis, emphysema, pulmonary hypertension.
- If DLCO is normal â Extraparenchymal problem (outside the lung). Think obesity, scoliosis, neuromuscular disease.
- If FEV1 and FVC are normal, but youâre short of breath â Check DLCO. It could be early ILD, pulmonary embolism, or heart issues.
The FVC/DLCO ratio is another clue. If itâs above 1.6, pulmonary hypertension is likely. If itâs below 1.3, youâre 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 âlung diseaseâ when itâs just anemia.
- Assuming normal spirometry = normal lungs: Missing early fibrosis or pulmonary embolism because they donât 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, donât forget technical factors. If you canât 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 isnât a bad result-itâs a bad test.
What Happens Next?
If your DLCO is low and spirometry is normal, your doctor might order a high-resolution CT scan. Thatâs the next step to see if thereâs 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. Thatâs when treatments like pirfenidone or nintedanib are considered.
And if youâre preparing for surgery-especially lung removal-DLCO helps predict if youâll tolerate it. If your DLCO is below 40%, your risk of complications rises.
Why These Tests Wonât Disappear
These arenât old-school tools fading into obsolescence. Theyâre evolving. In 2023, Mayo Clinic published a study using AI to predict pulmonary hypertension from DLCO patterns with 88% accuracy. Thatâs not science fiction-itâs 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.
Theyâre cheap, non-invasive, and give answers no X-ray or MRI can. You donât need a needle, radiation, or contrast. Just breath.
So if youâve been told your lungs are fine but you still canât catch your breath-ask for DLCO. It might be the test that finally explains why.
Meina Taiwo
December 20, 2025 AT 15:14DLCO is useless without hemoglobin correction. Seen too many patients get misdiagnosed because labs skip it.
Michael Ochieng
December 22, 2025 AT 12:08Man, I had normal spirometry for years but was gasping for air. Got my DLCO done and it was at 58%. Turned out to be early pulmonary fibrosis. They missed it because they didn't check my hemoglobin either. This post saved my life.
Jackie Be
December 23, 2025 AT 14:43SO THIS IS WHY I COULDN'T BREATHE AFTER WALKING UP THE STAIRS?? I THOUGHT I WAS JUST OUT OF SHAPE đ I ASKED FOR DLCO AND THEY LAUGHED AT ME. NOW I KNOW I WAS RIGHT. THANK YOU FOR THIS POST
Sandy Crux
December 23, 2025 AT 21:13While the content is technically accurate, the reliance on Global Lung Function Initiative equations remains problematic-particularly in non-Western populations where reference values are extrapolated from inadequate datasets. The assumption of universal applicability is a form of medical colonialism.
Hannah Taylor
December 24, 2025 AT 19:27they dont want you to know this. dlco is suppressed by big pharma because if you catch pulmonary embolism early you dont need their $8000/month drugs. also the machine is cheaper than an mri so they dont push it. my cousin got misdiagnosed for 3 years because they only did spirometry. theyre hiding it.
Peggy Adams
December 25, 2025 AT 12:41why do we even do these tests anymore? my insurance denied the dlco so i had to pay $300 out of pocket. then the doctor said it was probably just anxiety. i'm done.
Sarah Williams
December 25, 2025 AT 20:59Youâre not alone. I was told the same thing. Asking for DLCO was the best decision I ever made. Keep pushing. Your lungs deserve it.
Christina Weber
December 25, 2025 AT 22:04Incorrect. The Global Lung Function Initiative (GLI) equations are not merely a suggestion-they are the gold standard endorsed by the American Thoracic Society and the European Respiratory Society as of 2022. Any deviation constitutes substandard care.
John Hay
December 27, 2025 AT 13:02My dad had COPD. We never knew it was emphysema until his DLCO came back low. Thatâs when they started him on the right meds. This info matters.
Jon Paramore
December 28, 2025 AT 01:15DLCO < 75% with preserved FVC and normal FEV1/FVC is a classic restrictive pattern with impaired diffusion-likely ILD or early PH. The FVC/DLCO ratio >1.6 has >80% specificity for PH. Don't forget to correct for Hb and smoking status-both are confounders with effect sizes comparable to clinical pathology.
Cameron Hoover
December 29, 2025 AT 01:48I never thought breathing could be so complicated. I used to think if I wasnât wheezing, I was fine. Turns out I was just⌠quietly failing. This post gave me the courage to finally ask for the test. Thank you.
Stacey Smith
December 29, 2025 AT 18:31USA still leads in pulmonary diagnostics. Other countries donât even test DLCO routinely. Thatâs why our survival rates are better. Donât let bureaucrats cut these tests. They save lives.