FEV, or forced expiratory volume, is measured by blowing as hard and fast as you can into a device called a spirometer after taking the deepest breath possible. The spirometer records how much air you push out during specific time intervals, most commonly the first second (FEV1). The test is straightforward but technique-sensitive, meaning your results depend heavily on how well you perform the breathing maneuver and how you prepare beforehand.
What FEV Actually Measures
FEV quantifies the volume of air you can forcibly exhale within a set number of seconds after a full inhalation. The subscript tells you the time window: FEV1 is the volume expelled in one second, FEV6 is the volume expelled in six seconds. FEV1 is by far the most commonly used number and serves as the primary measure for gauging how severe an airflow obstruction is and whether treatment is working.
FEV6 has gained attention as a simpler alternative to another measurement called forced vital capacity (FVC), which requires you to blow out every last bit of air until your lungs are completely empty. FEV6 requires less physical effort, is more reproducible, and some experts now recommend using the FEV1/FEV6 ratio in place of the traditional FEV1/FVC ratio.
How the Test Is Performed
You sit upright, usually with a nose clip to prevent air from escaping through your nostrils. The technician hands you a mouthpiece connected to the spirometer. You wrap your lips tightly around the mouthpiece, breathe in as deeply as you possibly can, then blast the air out as hard and as fast as possible. You keep blowing until you either hit a plateau where no more air is coming out, or you’ve been exhaling for at least 15 seconds.
The explosive start matters. The device checks that the very beginning of your exhale was truly maximal, not hesitant or slow. If you cough during the first second, or your throat closes briefly, that attempt won’t count. The spirometer also checks for leaks around the mouthpiece and any signs of obstruction.
You’ll repeat this maneuver at least three times to get three acceptable readings. The two highest FEV1 values need to fall within 150 milliliters of each other (100 mL for children aged six and under). If your results are too far apart, you’ll keep going until they’re consistent. This repeatability check is what separates a reliable result from a lucky blow.
Preparing for the Test
What you do in the hours before spirometry directly affects your results. The National Heart, Lung, and Blood Institute recommends these specific guidelines:
- Food: No large meals within 2 hours of the test
- Exercise: No heavy physical activity for at least 30 minutes before
- Alcohol: None for at least 4 hours before
- Smoking: Do not smoke at all on the day of the test
If your test includes a bronchodilator challenge (to see if your airways open up with medication), you’ll also need to stop certain inhalers ahead of time. Short-acting rescue inhalers like albuterol should be withheld for 6 hours. Short-acting anticholinergic inhalers need a 12-hour window. Long-acting inhalers require 24 hours, and ultra-long-acting ones need 36 hours. Your doctor’s office will typically give you specific instructions when you schedule the appointment.
What the Numbers Mean
Your raw FEV1, measured in liters, doesn’t mean much on its own. A healthy 25-year-old man who is six feet tall will naturally have a much higher FEV1 than a healthy 70-year-old woman who is five feet tall. So your result gets compared to a predicted value based on your age, height, sex, and ethnicity. The Global Lung Initiative’s 2012 reference equations are the current standard, covering ages 3 through 95 across multiple ethnic groups.
Your result is then expressed as a percentage of predicted. An FEV1 of 95% predicted means your lungs are pushing out about as much air in one second as expected for someone with your characteristics. The ratio of FEV1 to FVC (or FEV6) adds another layer of information. A low ratio, where you’re moving a smaller-than-expected proportion of your total air volume in the first second, points to obstructive conditions like asthma or COPD. A normal ratio with low overall volumes suggests restrictive conditions, where the lungs can’t fully expand.
COPD Severity Staging
For COPD specifically, the Global Initiative for Chronic Obstructive Lung Disease (GOLD) uses FEV1 percentage of predicted to classify severity into four stages:
- Mild (Stage I): FEV1 at or above 80% predicted
- Moderate (Stage II): FEV1 between 50% and 79% predicted
- Severe (Stage III): FEV1 between 30% and 49% predicted
- Very severe (Stage IV): FEV1 below 30% predicted
The Bronchodilator Reversibility Test
When a doctor suspects asthma or wants to see how much airway narrowing is reversible, you’ll do the spirometry test twice in one visit. First, you blow baseline readings. Then you inhale a fast-acting bronchodilator and wait about 15 to 20 minutes. You repeat the test. If your FEV1 improves by at least 12% and at least 200 milliliters compared to baseline, your airflow obstruction is considered reversible. This pattern is a hallmark of asthma and helps distinguish it from COPD, where reversibility is typically more limited.
When FEV Testing Isn’t Safe
Spirometry requires generating significant pressure inside your chest, which can be risky in certain situations. The test is not performed if you have unstable heart conditions, a recent heart attack, a current respiratory infection, a recent collapsed lung, a pulmonary embolism, or a large aortic aneurysm (6 cm or greater). Coughing up blood, elevated pressure in the brain, and retinal detachment are also reasons to avoid the test entirely.
Recent surgeries on the abdomen, chest, brain, eyes, or ears and nose are relative contraindications, meaning the test might still be done if the benefit outweighs the risk, but your doctor needs to make that call. People who can’t follow the breathing instructions reliably, such as very young children or those with significant cognitive impairment, may not be able to produce valid results.
Home Spirometers and Portable Devices
Handheld digital spirometers have become widely available, and recent research shows they perform remarkably close to laboratory-grade equipment. A study comparing a portable spirometer to a high-quality lab device found correlation coefficients of 0.994 for FEV1 and 0.993 for FVC, with 96% of values falling within acceptable limits of agreement. The two devices also agreed on the diagnosis of abnormal spirometry and severity classification, with agreement scores above 0.87.
Perhaps more encouraging for home use, the same study found that non-specialists could obtain acceptable results with minimal guidance. Out of 30 volunteers using just tutorial videos and checklists, 28 achieved the highest quality grade. Portable spirometers are cheaper, require minimal maintenance, and don’t need dedicated testing space. They’re particularly useful for ongoing monitoring of conditions like asthma or COPD between clinic visits, though an initial diagnosis is still best confirmed with clinical-grade testing and professional interpretation.
The technique matters just as much at home as in a lab. Seal your lips completely around the mouthpiece, inhale until your lungs are absolutely full, and blast the air out without hesitation. Repeat until you get consistent readings. If your values start dropping significantly from your established baseline, that’s a signal to follow up with your care team.