A spirometry report comes down to three core numbers: FEV1, FVC, and the ratio between them. Once you know what each one measures and where the key cutoffs fall, the report stops looking like a foreign language. Your results will be shown as raw values (in liters) and as a “percent predicted,” which compares your lungs to what’s expected for someone of your age, height, sex, and ethnicity.
The Three Numbers That Matter
FVC (Forced Vital Capacity) is the total amount of air you can blow out after taking the deepest breath possible. It reflects lung size. On your report it will appear in liters and as a percent of predicted. A value at or above 80% of predicted is considered normal.
FEV1 (Forced Expiratory Volume in 1 Second) is how much of that air you can force out in the first second. It reflects how open your airways are. Like FVC, it’s reported in liters and as a percent predicted, with 80% or above being the normal benchmark.
FEV1/FVC Ratio is simply FEV1 divided by FVC. It tells you what fraction of your total air you can push out in that critical first second. A ratio above 0.70 (or 70%) is normal. This single number is the first thing clinicians look at to determine whether your breathing pattern is obstructive, restrictive, or normal.
What “Percent Predicted” Actually Means
Your raw numbers alone don’t tell the full story. A tall 25-year-old man will naturally have larger lung volumes than a shorter 70-year-old woman, so the report compares your results to a reference population matched to your age, height, sex, and ethnic background. The most widely used reference equations come from the Global Lung Function Initiative (GLI), which pooled data from over 70,000 healthy individuals aged 3 to 95. These equations account for the complex way lung function changes across childhood growth spurts, the adult plateau, and the gradual decline with aging.
Your percent predicted is your measured value divided by that reference value, multiplied by 100. An FEV1 of 95% predicted means your airways move 95% as much air in one second as expected for someone with your demographics. It’s worth noting that lung volumes differ proportionally between ethnic groups at the same height and age, but the FEV1/FVC ratio stays virtually the same across groups.
Step-by-Step Interpretation
Reading a spirometry report follows a logical sequence. Start with test quality, then move to the ratio, then to severity.
Step 1: Check the Quality Grade
Spirometry depends entirely on effort. If you didn’t blow hard enough, started too slowly, or stopped too early, the numbers won’t reflect your true lung function. Reports now use a letter grade from A to F. An A means the results are highly reliable. Anything below A means the results should be interpreted more cautiously. If your report shows a low grade, the test may need to be repeated.
Step 2: Look at the FEV1/FVC Ratio
This is the branching point. If the ratio is below 0.70, the pattern is obstructive, meaning air has trouble getting out. Conditions like COPD and asthma cause this. If the ratio is 0.70 or above, the pattern is either normal or restrictive, depending on whether FVC and FEV1 are also in the normal range.
One important caveat: the fixed 0.70 cutoff can overdiagnose obstruction in older adults. Research on never-smokers found that about 16 to 18% of people over age 70 fall below 0.70 without any lung disease, compared to about 7% of those in their 60s. Some guidelines suggest using the “lower limit of normal” (LLN), an age-adjusted threshold, instead. Your report may show both.
Step 3: Classify the Pattern
With the ratio in hand, the next step depends on what you see:
- Normal: FEV1/FVC ratio above 0.70, and both FEV1 and FVC at or above 80% of predicted.
- Obstructive: FEV1/FVC ratio below 0.70. The airways are narrowed, so air can’t flow out fast enough. FEV1 drops more than FVC does.
- Restrictive (suggested): FEV1/FVC ratio at or above 0.70, but FVC is below 80% of predicted. This hints that the lungs can’t fully expand. Spirometry alone can’t confirm restriction; a full lung volume test (measuring total lung capacity) is needed.
- Mixed: FEV1/FVC ratio below 0.70 and total lung capacity below 80% of predicted. Both obstruction and restriction are present.
Severity of Obstruction
When an obstructive pattern is identified, the next question is how severe it is. Severity is graded using FEV1 percent predicted, not the ratio. Two commonly used scales exist. The traditional five-tier system breaks it down this way:
- Mild: FEV1 above 70% of predicted
- Moderate: FEV1 between 60% and 69%
- Moderately severe: FEV1 between 50% and 59%
- Severe: FEV1 between 35% and 49%
- Very severe: FEV1 below 35%
The GOLD staging system used specifically for COPD uses slightly different cutoffs: stage 1 is 80% or above, stage 2 is 50 to 79%, stage 3 is 30 to 49%, and stage 4 is below 30%. If your report references GOLD stages, that’s the scale being used.
Bronchodilator Response
Many spirometry tests include a second round of blowing after you inhale a bronchodilator (typically a fast-acting inhaler). This “pre and post” comparison reveals whether your airway narrowing is reversible. A significant response is defined as an improvement in FEV1 or FVC of at least 12% and at least 200 mL. Meeting both thresholds matters; a small percentage change on a very low baseline doesn’t count, and neither does a large percentage change that amounts to only a trivial volume.
A strong bronchodilator response is a hallmark of asthma, where the airways tighten and relax. In COPD, the response is typically smaller or absent because the airway damage is more structural. That said, the distinction isn’t absolute. Some people with COPD do show partial reversibility, and some people with asthma don’t respond dramatically on a given day.
Why Preparation Affects Your Numbers
If your test included bronchodilator testing, you were likely asked to stop certain medications beforehand. Short-acting inhalers like albuterol need to be withheld for 6 hours before the test. Short-acting anticholinergic inhalers require 12 hours. Long-acting bronchodilators need 24 hours, and ultra-long-acting agents like tiotropium require 36 hours. Using these medications too close to your test can mask the true degree of airway narrowing and make your baseline results look better than they are.
Beyond medications, effort matters enormously. The test requires a maximal blast of air sustained for at least 6 seconds (sometimes longer). Hesitating at the start, coughing mid-blow, or stopping early all produce numbers that underestimate your lung function. That’s why the quality grade on the report is the first thing worth checking.
Reading the Flow-Volume Loop
Most spirometry reports include a graph called the flow-volume loop. It plots airflow speed (vertical axis) against the volume of air exhaled (horizontal axis). In a healthy person, the curve rises sharply to a peak and then slopes down in a fairly straight line. In obstructive disease, the descending portion scoops inward, forming a concave shape, because airflow slows down as the narrowed airways collapse during exhalation. In restrictive disease, the overall shape looks normal but the entire loop is smaller, reflecting reduced lung volume. Comparing your loop to the predicted loop (often shown as a dotted line on the same graph) gives you a visual sense of how far your results deviate from expected.
If your report shows multiple loops from repeated efforts, the one with the highest combined FEV1 and FVC is typically the one used for interpretation. The others are there to demonstrate consistency. When the best efforts are close to each other, it’s a sign the test was performed well.