Lung function is most commonly measured with a simple breathing test called spirometry, which takes about 15 minutes and can detect problems long before symptoms become serious. Beyond spirometry, there are several other tools ranging from handheld devices you can use at home to advanced hospital-based tests that map your lung capacity in detail. The right test depends on what your doctor is looking for.
Spirometry: The Standard Test
Spirometry is the foundation of lung function testing. You breathe into a mouthpiece connected to a machine called a spirometer, which measures how much air you can move and how quickly you can move it. It’s used to diagnose conditions like asthma, COPD, and pulmonary fibrosis, and to track how well treatments are working over time.
The test captures two key numbers:
- Forced vital capacity (FVC): the total amount of air you can blow out after taking the deepest breath possible.
- Forced expiratory volume in one second (FEV1): how much of that air you can push out in the first second.
Your results are compared to predicted values for someone of your same height, age, and sex, which are the three biggest factors that determine normal lung capacity. A score at or above 80% of your predicted value is generally considered normal. Below that, your provider will look at the pattern of your results to figure out what type of problem might be present.
What Your Results Mean
The relationship between FEV1 and FVC tells your doctor a lot. Providers look at the ratio of FEV1 to FVC to distinguish between two broad categories of lung disease.
A low FEV1/FVC ratio (generally below 0.70, though age-adjusted cutoffs are now preferred) signals an obstructive pattern. This means air has trouble getting out of your lungs, which is what happens in asthma and COPD. Your lungs may hold a normal amount of air, but the airways are narrowed, so you can’t exhale it quickly.
When both FEV1 and FVC are reduced but the ratio between them stays normal or even goes up, that suggests a restrictive pattern. This means your lungs can’t fully expand, which occurs in conditions like pulmonary fibrosis or chest wall problems. Confirming a restrictive diagnosis requires additional lung volume measurements beyond basic spirometry.
Peak Flow Meters for Home Monitoring
If you have asthma, your doctor may give you a peak flow meter, a small handheld device that measures how fast you can push air out of your lungs. It doesn’t replace spirometry, but it gives you a daily snapshot of how your airways are doing.
To use one: stand or sit up straight (stay consistent each time), slide the marker to the bottom of the scale, take the deepest breath you can, seal your lips around the mouthpiece, and blow out as hard and fast as possible. You’ll typically do this twice a day for two weeks to establish your “personal best” number, which becomes your baseline.
Results follow a traffic light system based on your personal best:
- Green zone (80% to 100%): your asthma is well controlled.
- Yellow zone (50% to 80%): your airways are narrowing and you may need to adjust treatment.
- Red zone (below 50%): you need emergency care.
Pulse Oximetry: Checking Oxygen Levels
A pulse oximeter clips onto your fingertip and uses light to estimate how much oxygen your blood is carrying. It doesn’t measure lung capacity like spirometry does, but it reveals whether your lungs are doing their core job of getting oxygen into your bloodstream. Normal oxygen saturation falls between 95% and 100% for most healthy people, though it can run lower in those with chronic lung or heart conditions.
Several factors can throw off the reading. Nail polish, cold fingers, poor circulation, tobacco use, and skin thickness all reduce accuracy. The FDA has also noted that pulse oximeters can be less accurate on people with darker skin pigmentation, sometimes overestimating oxygen levels. For the most reliable reading, make sure your hand is warm and relaxed, remove nail polish, and hold your hand below heart level.
Advanced Clinical Tests
When spirometry raises questions it can’t fully answer, your doctor may order more specialized tests.
Lung Diffusion Testing (DLCO)
This test measures how efficiently oxygen passes from your lungs into your blood. You breathe in a gas mixture containing a tiny, harmless amount of carbon monoxide, hold your breath briefly, then exhale into the machine. The machine calculates how much carbon monoxide your blood absorbed. Since carbon monoxide crosses into the blood the same way oxygen does, the result reveals how well your lung tissue is working at the microscopic level. A low DLCO can point to conditions like emphysema, pulmonary fibrosis, or blood vessel problems in the lungs, even when spirometry looks relatively normal.
Body Plethysmography
This test measures total lung volume, including the air that stays trapped in your lungs after you exhale (which spirometry can’t capture). You sit inside a clear, phone booth-sized chamber and breathe against a closed shutter while sensors track pressure changes. It’s especially useful for confirming restrictive lung disease and for detecting air trapping in obstructive conditions.
How to Prepare for Lung Function Tests
Preparation matters because eating, smoking, or using inhalers before the test can skew your results. The National Heart, Lung, and Blood Institute recommends these guidelines for spirometry:
- Food: avoid a large meal within two hours of the test.
- Smoking: do not smoke on the day of the test.
- Short-acting inhalers (like albuterol): stop at least 6 hours before.
- Long-acting inhalers: stop 24 to 36 hours before, depending on the medication.
Diffusion testing is stricter: no smoking or alcohol on the day of the test. If you’re scheduled for an exercise challenge test, avoid caffeine for 12 hours and food for 3 hours beforehand, and wear comfortable clothes and sneakers. Your doctor’s office should give you specific instructions, but knowing the basics ahead of time helps you avoid having to reschedule.
Smartphone Apps and Newer Technology
Researchers are exploring whether a smartphone microphone can estimate lung function by analyzing the sound of a forced exhale. Deep learning models have shown a promising correlation between breathing sounds and traditional spirometry values. However, current error margins of 20% to 35% fall well short of the 5% to 10% accuracy required for clinical spirometry. The technology measures sound waves rather than actual airflow, so it’s fundamentally different from a spirometer.
These tools may eventually become useful for screening in areas without access to medical equipment, but they aren’t reliable enough to replace a proper lung function test. If you’ve seen an app claiming to measure your lung capacity, treat the results as a rough estimate at best.