Lung Diffusion Testing: Purpose, Procedure, and Results

If you’ve ever wondered how doctors can tell whether your lungs are actually moving oxygen into your blood (and not just making heroic “breathing effort” noises), you’re in the right place.
A lung diffusion testmost commonly called a DLCO testchecks how efficiently gases cross from the tiny air sacs in your lungs into the bloodstream.
Think of it like a “customs checkpoint” for oxygen: the test helps reveal whether gas exchange is smooth and speedy… or stuck in a long line with one overwhelmed TSA agent.

This article breaks down what lung diffusion testing is, why it’s ordered, what happens during the procedure, how results are reported, and what patterns can meanusing real-world examples, plain English, and just enough humor to keep things breathable.
(You’ll still want a clinician to interpret your results in context, because lungs love nuance.)

What Is Lung Diffusion Testing (DLCO)?

Lung diffusion testing measures how well gases move from the air inside your lungs into your blood. The most common method measures the diffusing capacity of the lung for carbon monoxide, abbreviated DLCO.
That sounds alarming until you learn the dose is tiny and the test is designed around a clever science trick.

Why carbon monoxide is used (and why it’s not as scary as it sounds)

In a DLCO test, you inhale a harmless mixture containing a very small amount of carbon monoxide (CO) plus a “tracer” gas.
CO is useful because it binds strongly to hemoglobin in red blood cells, so it behaves like a stand-in for oxygen transfer.
The testing equipment compares how much CO went in versus how much comes back out to estimate how well gas is crossing the alveolar-capillary membrane.
In other words: your lungs borrow a microscopic “measuring tape gas” for one breath, then return it immediately.

Why Would Someone Need a Lung Diffusion Test?

DLCO is often ordered as part of a broader set of pulmonary function tests (PFTs). It helps clinicians answer questions like:
“Is the shortness of breath coming from airflow issues, scarring, blood vessel problems, anemia, or something else?”

Common reasons clinicians order DLCO

• Unexplained shortness of breath (especially with normal or near-normal spirometry).
• Suspected interstitial lung disease (ILD) or pulmonary fibrosis.
• Emphysema/COPD evaluation (DLCO often drops when alveolar surface area is damaged).
• Possible pulmonary hypertension or other pulmonary vascular disease.
• Pre-operative or pre-treatment assessment (for some surgeries or lung-toxic medications).
• Monitoring over time to track progression or response to treatment.
• Clarifying restrictive patterns (lung stiffness vs. chest wall/neuromuscular causes).

What DLCO can add beyond spirometry

Spirometry tells you about airflow (how fast air moves out). Lung volumes tell you about capacity (how much air fits).
DLCO tells you about gas exchange (how well oxygen can “get into the blood neighborhood”).
That’s why DLCO is especially valuable when symptoms feel out of proportion to basic breathing tests.

How the DLCO Test Works

DLCO is usually performed using the single-breath method. The idea is simple:
take in a special gas mixture, hold it briefly, then exhale so the machine can analyze how much was absorbed.

The basic physics (no lab coat required)

Gas exchange depends on several factors:

• Surface area (more healthy alveoli = more “space” to exchange gases).
• Membrane thickness (scarring/inflammation can make the barrier thicker and slower).
• Blood flow in lung capillaries (reduced flow can lower transfer).
• Hemoglobin level (less “carrying capacity” can reduce measured diffusion).
• Ventilation distribution (how evenly air reaches different lung regions).

Preparing for Lung Diffusion Testing

Preparation is usually minimal, but small details can noticeably affect results. Follow your testing lab’s instructions, and if anything is unclear, askthis is not the time to freestyle.

Typical prep steps

• Avoid smoking for several hours before the test (smoking can raise carboxyhemoglobin and alter readings).
• Avoid heavy meals right before testing (a very full stomach can make deep breaths uncomfortable).
• Ask about inhalers or bronchodilators: some labs want you to hold certain meds, others don’t.
• Avoid strenuous exercise right beforehand unless your lab says it’s fine.
• Wear comfortable clothing that doesn’t restrict breathing.

If you use supplemental oxygen, your lab may give instructions about whether to pause it briefly before the testonly do this if you’re told it’s safe for you.
Safety always beats “perfect data.”

The Procedure: What Happens During a DLCO Test

DLCO testing is noninvasive and typically takes only a few minutes, though the full PFT appointment can take longer if you’re doing spirometry and lung volumes too.

Step-by-step: the classic single-breath DLCO maneuver

1. You’ll sit upright and wear a nose clip so all air goes through the mouthpiece.
2. You exhale fully to empty your lungs as much as possible.
3. You inhale quickly and deeply from the machine to near-full lung capacity.
4. You hold your breath briefly (often around ~10 seconds).
5. You exhale steadily back into the mouthpiece while the machine collects a sample for analysis.
6. The maneuver is repeated a few times to ensure quality and consistency.

What it feels like

Most people describe DLCO as “weird but fine.”
The mouthpiece may feel tight, the nose clip is mildly annoying, and the breath-hold can feel long if you’re anxious (or if you just sprinted up the stairs to the lab).
You might feel slightly lightheaded for a moment after a big inhale and breath-holdtell the technician if you do.

Safety and risks

DLCO is considered very safe for most people. Potential issues are usually minor and short-lived:
lightheadedness, coughing, or brief breathlessness during repeated maneuvers.
People with certain unstable heart or lung conditions may need extra caution, so always tell the lab about recent chest pain, recent lung collapse, significant dizziness, or if you feel unwell that day.

Understanding DLCO Results

DLCO results are typically reported as:
(1) an absolute number (often mL/min/mmHg) and (2) a percent of predicted based on reference equations that account for factors like age, height, and sex.
Many labs also include z-scores and a lower limit of normal (LLN), which helps avoid over-calling “abnormal” when you’re near a cutoff.

DLCO vs. VA vs. KCO: the trio that tells a better story

DLCO interpretation often improves when you look at related values:

• VA (alveolar volume): an estimate of the lung volume participating in the test.
• KCO (transfer coefficient): essentially DLCO “per unit lung volume” (often described as DLCO/VA).

Why this matters: a low DLCO can happen because the lungs’ exchange surface is damaged or thickened,
or because less lung volume participated in the maneuver. VA and KCO help your clinician separate those possibilities.

What’s considered “normal”?

“Normal” depends on the reference equations used and your personal factors. Many labs consider DLCO normal if it’s above the LLN and/or within a typical percent-predicted range.
If you see numbers like “% predicted,” remember: that’s not a grade on your lungs’ report cardit’s a comparison to a healthy reference group.

What Low DLCO Can Mean

A low DLCO suggests less efficient gas transfer from the lungs into the blood.
That can happen when the exchange surface area is reduced, the membrane is thickened, blood flow is reduced, or hemoglobin is low.

Common causes of low DLCO

• Emphysema (COPD): damaged alveoli reduce surface area.
• Interstitial lung disease (pulmonary fibrosis, inflammatory ILD): thickened/scarred membrane slows transfer.
• Pulmonary hypertension or pulmonary vascular disease: reduced capillary blood volume/flow.
• Anemia: less hemoglobin available to bind gas (DLCO may be adjusted if hemoglobin is measured).
• Prior lung resection or significant loss of functioning lung tissue.
• Some chronic heart conditions that affect pulmonary blood flow can also contribute.

A practical example

Imagine two people with shortness of breath:

• Person A has COPD with emphysema. Spirometry shows obstruction, and DLCO is lowthis fits the pattern of reduced surface area.
• Person B has near-normal spirometry but a notably low DLCO. That raises suspicion for pulmonary vascular issues (like pulmonary hypertension) or early interstitial lung diseaseespecially if imaging or symptoms support it.

DLCO doesn’t diagnose a disease by itself, but it can point clinicians toward the most likely “bucket” of problems and help decide what to do next (imaging, labs like hemoglobin, echocardiogram, specialty referral, etc.).

What High DLCO Can Mean

A high DLCO is less common and often less worrisome than a low DLCO, but it can still be clinically meaningful.
A higher value can occur when pulmonary blood volume is increased or when certain physiologic conditions boost gas uptake.

Common causes of high DLCO

• Asthma (DLCO can be normal or high, especially compared with COPD).
• Obesity (sometimes associated with higher pulmonary blood volume and higher DLCO).
• Polycythemia (more red blood cells can increase gas uptake).
• Alveolar hemorrhage (blood in alveoli can raise CO uptakethis is uncommon and typically comes with significant symptoms).
• Left-to-right cardiac shunt (rare; increases pulmonary blood flow).

If DLCO is high, clinicians usually interpret it alongside symptoms and the rest of the PFTs. Sometimes the most important takeaway is simply:
“This doesn’t look like emphysema,” especially in someone with wheezing and shortness of breath.

Factors That Can Skew Results (Even When Your Lungs Are Fine)

DLCO is sensitivewhich is great when you’re diagnosing subtle disease, and less great when your morning includes smoking, sprinting, and a bagel the size of your head.
Several factors can influence readings:

Common “false low” or “false high” influences

• Smoking recently (can change CO binding dynamics and baseline carboxyhemoglobin).
• Anemia or polycythemia (hemoglobin level changes gas-carrying capacity).
• Altitude/barometric pressure differences (labs standardize measurements, but context matters).
• Poor test effort (not inhaling fully, short breath-hold, leaks around the mouthpiece).
• Supplemental oxygen right before the test (may affect some measurements and interpretation depending on protocol).
• Recent respiratory infection or flare that temporarily changes airways and ventilation distribution.

This is why good labs repeat maneuvers and report quality indicators. If a result surprises your clinician, they may repeat testing or interpret it cautiously rather than jumping to conclusions.

DLCO in Context: How Clinicians Combine It With Other Tests

A DLCO number is most powerful when paired with spirometry, lung volumes, imaging, and clinical history.
Here are a few classic “pattern matches” clinicians use:

Pattern 1: Obstruction + low DLCO

Often points toward emphysema-predominant COPD, where airflow is limited and alveolar surface area is reduced.

Pattern 2: Restriction + low DLCO

Suggests interstitial lung disease or another process affecting the alveolar-capillary membrane.
Clinicians often correlate this pattern with CT imaging, symptoms, and sometimes autoimmune evaluations.

Pattern 3: Normal spirometry + low DLCO

Raises suspicion for pulmonary vascular disease, early ILD, anemia, or other causes.
Depending on the full picture, follow-up might include hemoglobin testing, imaging, or a cardiac evaluation.

Pattern 4: Obstruction + normal/high DLCO

Can fit better with asthma than emphysema (though real life is messy, and overlap exists).

Frequently Asked Questions

How long does the test take?

The DLCO portion is usually quickoften just a few minutesthough your appointment may be longer if you’re doing a full PFT panel.

Can a DLCO test diagnose a specific condition?

Not by itself. DLCO is best viewed as a strong clue. Diagnoses come from combining DLCO with your symptoms, exam, spirometry, lung volumes, imaging, and sometimes blood tests.

What should I do if my DLCO is low?

Don’t panic, don’t self-diagnose, and definitely don’t start “Googling aggressively” at 2 a.m. without context.
Ask your clinician what the result means for you, whether hemoglobin correction was applied, and what next steps (if any) are recommended.

Will I need repeat testing?

Possibly. DLCO is often repeated to monitor disease progression or response to treatment, or to confirm a surprising result.
Many clinicians focus on trends in the absolute value over time, not just percent predicted.

Real-World Experiences: What People Often Notice (About )

If you’re heading into lung diffusion testing, the unknown can feel bigger than the test itself. So here’s what people commonly reportno drama, no medical theater, just real-life vibes.

First, there’s the “equipment moment.” You’ll sit down, get a nose clip, and meet the mouthpiece. The mouthpiece looks like it belongs on a sci-fi set, but in practice it’s mostly just… a mouthpiece.
People often say the nose clip is the funniest partnot because it hurts, but because it makes you instantly aware of how much you normally rely on your nose to do its job quietly in the background.

Next comes the breathing choreography. The technician will coach you through the steps like a calm, friendly conductor: breathe out all the way, big inhale, hold, then exhale.
The hold can feel surprisingly long if you’re nervous. A common tip people discover on their own (and then wish they’d known earlier): relaxing your shoulders and unclenching your jaw makes the breath-hold feel easier.
Tension is basically the worst travel buddy for any breathing test.

Some people feel a little lightheaded for a moment after the big inhale and holdespecially if they’re already short of breath or if they’ve been breathing fast from anxiety.
That’s not unusual, and the lab staff is used to it. Most folks recover quickly with a few normal breaths.
If you’re worried about feeling dizzy, tell the technician upfront; they can pace things, give you breaks, and make sure you feel steady between maneuvers.

Then there’s the “am I doing this right?” phase. Many people feel oddly competitive with themselves: “Was that inhale deep enough?”
Here’s the comforting part: the equipment and the technician are built for exactly this. You’re not expected to be a professional breather (even if you’ve been doing it your whole life).
The test is repeated specifically to get consistent, high-quality measurements, and the tech can spot leaks or suboptimal efforts and help you correct them.

Afterward, most people feel totally normal and go right back to their day. The most common lingering sensation is simply mild fatigue from repeated deep breathinglike you did a tiny workout that only involved your diaphragm.
Emotionally, the biggest “after effect” tends to be anticipation: waiting to hear what the numbers mean.
If you’re in that boat, consider writing down two or three questions for your follow-up visit, such as: “Was my test quality good?” “Was hemoglobin considered?” and “How do my results fit with my symptoms and imaging?”
It’s a small move that can make the conversation clearerand keep you from spiraling into online rabbit holes where every symptom equals a rare disease and every cough is a plot twist.

Conclusion

Lung diffusion testing (DLCO) is a fast, noninvasive way to measure how efficiently gases move from your lungs into your bloodstream.
It’s especially useful for sorting out the “why” behind shortness of breath, tracking conditions like interstitial lung disease or COPD, and helping clinicians understand whether gas exchange is being limited by lung tissue, blood flow, or other factors.
The key to making DLCO meaningful is context: test quality, preparation, hemoglobin, and the rest of your pulmonary function results all matter.
If your DLCO is abnormal, treat it like a valuable cluenot a final verdictand review it with a clinician who can connect the dots.