A spirometer is a medical device that measures how much air you can breathe in or out, and how quickly. It connects to a mouthpiece and records your airflow, giving doctors a detailed picture of how well your lungs are working. Spirometers come in two distinct forms: diagnostic devices used in clinics to detect lung disease, and simpler incentive spirometers given to patients after surgery to keep their lungs active during recovery.
How a Spirometer Works
At its core, a spirometer tracks the movement of air over time. Diagnostic spirometers connect to a computerized machine that records both the volume of air you exhale and the speed at which it leaves your lungs. The device converts your breath into a digital signal, sampling your airflow many times per second to create a precise graph of each breath. From that graph, the machine calculates several key numbers your doctor uses to assess lung function.
Older spirometers used mechanical bells or pistons that physically moved as you exhaled into them. Modern versions use electronic sensors called pneumotachographs, which measure airflow by detecting tiny pressure changes as air passes through the device. The result is the same: a detailed recording of how your lungs perform during forced breathing.
Diagnostic Spirometers vs. Incentive Spirometers
These two devices share a name but serve very different purposes.
A diagnostic spirometer is the device used in a doctor’s office or pulmonary function lab. You blow into it as hard and fast as you can, and the machine analyzes the pattern of your exhalation to detect conditions like asthma, COPD, or pulmonary fibrosis. It produces precise measurements and generates graphs that help classify the type and severity of any lung problem.
An incentive spirometer is a simple, handheld plastic device you’ll typically receive after surgery. Instead of measuring exhalation, it measures how much air you can inhale. When you breathe in through the mouthpiece, a piston rises inside the chamber, showing you the volume of your breath. The visual feedback encourages you to take slow, deep breaths, which helps prevent lung complications during recovery. Deep breathing moves secretions out of the airways and reopens lung spaces that may have collapsed while you were under anesthesia or lying in bed.
Incentive spirometers come in two styles. Volume-oriented devices use a piston to show the total air inhaled, while flow-oriented devices contain small balls in separate chambers that rise when you breathe in fast enough. Volume-oriented models tend to be more effective because they encourage deeper use of the diaphragm rather than the upper chest muscles. Your provider will typically set a target volume for you to work toward, and the standard recommendation is 10 to 15 breaths every one to two hours after surgery.
What a Spirometry Test Measures
When you take a diagnostic spirometry test, the machine captures several values. The two most important are:
- FVC (Forced Vital Capacity): The total volume of air you can force out of your lungs after breathing in as deeply as possible.
- FEV1 (Forced Expiratory Volume in 1 Second): How much of that air you can push out in the first second alone.
The ratio between these two numbers is the single most useful value for detecting airflow obstruction. In healthy lungs, you can typically exhale 75% to 85% of your total air in that first second. If the ratio drops below normal, it suggests something is narrowing or blocking the airways, as seen in asthma or COPD. If both FEV1 and FVC are reduced but the ratio stays normal or even increases, that pattern points toward a restrictive problem, where the lungs can’t fully expand due to scarring, chest wall abnormalities, or other conditions.
Doctors also use spirometry to track treatment. If your FEV1 improves by 10% or more after using a bronchodilator (the inhaled medication that opens airways), that’s considered a positive response and helps guide your treatment plan.
What the Test Feels Like
A spirometry test is straightforward but physically demanding in a brief, intense way. You sit upright, place your lips tightly around a disposable mouthpiece, breathe in as deeply as you can, then blast the air out as hard and fast as possible until your lungs feel completely empty. The technician will coach you through each attempt.
You’ll repeat this at least three times to ensure consistent results. The technician records the best values from those attempts. The whole process usually takes about 15 to 30 minutes, though the actual blowing takes only seconds each time. You may feel lightheaded, dizzy, or tired from the repeated deep breaths, and the forceful exhalation can trigger coughing. These are normal reactions. You can ask for a break between attempts if you need one.
How Results Are Interpreted
Your spirometry numbers are compared against reference values based on your age, height, and sex using equations developed from large global datasets. The current international standard, established by the Global Lung Function Initiative, uses a race-neutral approach to generate these predicted values.
Rather than simply comparing your result to a percentage of the predicted value (an older method now considered less accurate), current guidelines recommend using a statistical measure called a Z-score. This score tells your doctor how far your measurement falls from what’s expected for someone of your age and size. A Z-score below negative 1.64 is considered abnormal, meaning your result falls below the bottom 5% of healthy individuals.
Z-scores also classify severity:
- Mild impairment: Z-score between negative 1.65 and negative 2.5
- Moderate impairment: Z-score between negative 2.51 and negative 4.0
- Severe impairment: Z-score below negative 4.1
This approach matters because lung function naturally declines with age. Older methods using a fixed cutoff ratio of 0.70 for FEV1/FVC tended to overdiagnose obstruction in older adults (especially men) and miss it in younger people (especially women). The Z-score method accounts for this natural variation, giving a more accurate picture regardless of your age.
Conditions Spirometry Helps Detect
Spirometry is the primary tool for diagnosing and monitoring obstructive lung diseases like asthma and COPD. It picks up the characteristic pattern of air getting trapped behind narrowed airways. For restrictive lung diseases, where the lungs can’t fully expand due to scarring or other damage, spirometry serves as a useful screening tool, though confirming a restrictive pattern typically requires additional testing that measures total lung capacity.
Beyond diagnosis, spirometry tracks disease progression over time and helps determine whether medications are working. It’s also used in occupational health settings to monitor workers exposed to dust, chemicals, or other airborne hazards that can damage the lungs. The best cutoff for detecting a restrictive pattern in these settings is an FVC below 70% of predicted, though the Z-score method provides more reliable results across different populations.