A spirometry report boils down to a handful of numbers and a couple of graphs, and once you know what each one represents, the whole document becomes surprisingly readable. The three values that carry the most weight are the FVC (forced vital capacity, or the total air you can blow out after a full breath), the FEV1 (how much of that air comes out in the first second), and the ratio between them. Those three numbers, compared against predicted values for your age, height, sex, and ethnicity, tell a clinician whether your airways are narrowed, whether your lungs can hold a normal volume of air, or both. But the numbers alone do not tell the whole story, and understanding what surrounds them on the page helps you make sense of what your doctor is actually looking at.
The Three Core Numbers
When you open a spirometry report, the first columns you will see are your measured values alongside “predicted” values and a “percent predicted” or a Z-score. The predicted value is what someone of your same age, height, sex, and ethnic background would typically produce with healthy lungs. Your percent predicted is simply your measured value divided by that predicted number, expressed as a percentage. A Z-score does essentially the same thing but accounts for how spread out normal values are in the population, so it handles age- and height-related variation more evenly.
FVC tells you the total volume of air you forced out of your lungs. FEV1 tells you how much came out during the first second. The FEV1/FVC ratio tells you what fraction of your total breath you could get out quickly. In healthy lungs, most of the air exits in that first second, so a normal ratio is usually above about 0.70 to 0.80 in younger adults, trending slightly lower with age.
These three values are the backbone of every interpretation. A low ratio with a preserved or low FVC points toward obstruction. A normal or high ratio with a low FVC raises the question of restriction. And the severity of any abnormality is graded primarily from the FEV1. The ATS/ERS grading system, which many labs still use, categorizes severity of an obstructive defect by FEV1 percent predicted: mild is above 70%, moderate 60–69%, moderately severe 50–59%, severe 35–49%, and very severe below 35%.1Libyan Journal of Medicine. Defining and grading an obstructive ventilatory defect (OVD)
Predicted Values and Why They Matter
The predicted value on your report is not a fixed target everyone shares. It comes from reference equations, which are statistical models built on large studies of healthy people. Different reference sets can produce different predicted values, and this has real consequences for your interpretation. The Global Lung Initiative (GLI) 2012 equations are now the most widely recommended worldwide, but older equations remain in use at many labs. A study comparing GLI equations against older Canadian reference values found that the lower limit of normal for FVC and FEV1 differed significantly between the two, and those differences varied across ethnic groups.2BMJ Open Respiratory Research. Changes in interpretation of spirometry by implementing the GLI 2012 reference equations
If your report lists “LLN” (lower limit of normal), that is the cutoff below which a value is considered abnormally low, typically the fifth percentile of the healthy reference population. A result just above the LLN is normal; just below, abnormal. The older convention of using a fixed 0.70 cutoff for the FEV1/FVC ratio is still widely seen, especially in COPD guidelines, but it tends to over-diagnose obstruction in older adults (whose ratio naturally drifts lower with age) and under-diagnose it in younger adults. The GLI Z-score approach handles this better because it adjusts for age-related spread in normal values.
Recognizing an Obstructive Pattern
An obstructive pattern shows up when your FEV1/FVC ratio falls below the lower limit of normal. The airflow is being slowed, so a smaller fraction of your total breath gets out in the first second. This is the hallmark of conditions like COPD, asthma, and bronchiectasis. Your FVC might be normal, mildly reduced, or even low (because air can become trapped in obstructed lungs), but the defining feature is always that reduced ratio.
Once obstruction is confirmed, severity is graded by FEV1. There has been longstanding debate about the best way to do this. The traditional GOLD staging for COPD uses FEV1 percent predicted, but this measure can be influenced by demographic factors and does not fully account for differences in lung size. A newer classification called STAR (Staging of Airflow obstruction by Ratio) stages severity directly by the FEV1/FVC ratio itself, in bands of 0.60–0.70, 0.50–0.60, 0.40–0.50, and below 0.40. In large cohorts, STAR provided a more uniform gradation of disease severity and differentiated symptoms, exacerbation risk, and prognosis at least as well as the traditional GOLD scheme, while being less sensitive to race and ethnicity.3PubMed Central. FEV1/FVC Severity Stages for Chronic Obstructive Pulmonary Disease You may or may not see STAR on your report yet, but it signals where the field is heading.
When the Report Suggests Restriction
A restrictive pattern on spirometry looks like a low FVC with a normal or even elevated FEV1/FVC ratio. The lungs seem smaller than they should be, so less air goes in and less comes out, but the airways themselves are not narrowed. This pattern can show up in interstitial lung diseases, chest wall abnormalities, obesity, or neuromuscular disorders.
Here is the catch: spirometry alone cannot confirm restriction. A study directly measuring how well spirometry predicts true lung restriction found that fewer than 60% of patients who had a classic restrictive spirometric pattern actually turned out to have reduced total lung capacity when it was measured directly.4PubMed. How accurate is spirometry at predicting restrictive pulmonary impairment? The remaining patients had other explanations for their low FVC, such as air trapping from obstructive disease that was not obvious from the spirometry numbers alone. So if your report flags a restrictive pattern, expect your clinician to order full lung volume measurements (typically by body plethysmography) before concluding that you actually have restricted lung expansion.
Mixed Patterns and Why They Are Tricky
Sometimes a report shows a low FEV1/FVC ratio and a low FVC at the same time. This looks like both obstruction and restriction occurring together, and it does happen, for example in a person with COPD who also has a chest wall deformity. But the appearance of a mixed pattern on spirometry is often misleading. In one study of 58 patients whose spirometry showed a mixed pattern, only about a third actually had confirmed restriction when total lung capacity was measured. The majority had pure obstructive disease; their FVC was low because air was trapped behind narrowed airways, not because the lungs were physically small.5Tuberculosis and Respiratory Diseases. How many patients with mixed spirometric pattern really have restrictive disorders? The takeaway: a mixed pattern on paper is a prompt for further testing, not a final diagnosis.
The Bronchodilator Response Section
Many spirometry reports include pre- and post-bronchodilator results. You blow into the spirometer, then inhale a short-acting bronchodilator (usually albuterol or salbutamol), wait about 15 minutes, and blow again. The report will show the change in FEV1 and FVC in both absolute volume and percentage. A positive bronchodilator response is generally defined as an increase of at least 12% and at least 200 mL in either FEV1 or FVC.6PubMed Central. Spirometry and Bronchodilator Test
This test matters because it helps distinguish between different causes of airflow obstruction. In asthma, the airways are often highly reversible, meaning the bronchodilator opens them up substantially. In COPD, obstruction tends to be more fixed. But this distinction is far from clean-cut. Research has shown that the bronchodilator response in FEV1 is neither sensitive enough nor specific enough to reliably tell asthma from COPD on spirometric grounds alone.7PubMed. Is the short-term response to inhaled beta-adrenergic agonist sensitive or specific for distinguishing between asthma and COPD? Many COPD patients show partial reversibility, and some people with asthma do not reverse much on a single test day. Clinicians weigh the bronchodilator response alongside symptoms, history, and other tests rather than relying on it as a standalone diagnostic marker. Postbronchodilator spirometry is always required to confirm a COPD diagnosis, and prebronchodilator testing is standard when evaluating asthma.8PubMed Central. Importance of distinguishing between asthma and chronic obstructive pulmonary disease in primary care
Reading the Graphs
Most reports include two graphs. The volume-time curve plots the total air expired over time. A healthy curve rises steeply at first (the fast initial burst of air) then levels off to a plateau, showing that expiration is complete. The flow-volume loop plots how fast air moves at different points during a full forced breath out and back in, creating a characteristic shape: a sharp peak of flow early in expiration, then a gradual decline, followed by a smoother inspiratory limb below the axis.
The shape of the flow-volume loop is diagnostic in itself. In obstructive disease, the expiratory limb becomes “scooped out” or concave because flow drops off more steeply after the initial peak. In restriction, the loop looks like a miniature version of normal, smaller but with a preserved shape. Upper airway obstructions produce distinctive patterns: a fixed lesion (something blocking the trachea that does not move with breathing) flattens both the inspiratory and expiratory limbs into plateaus. A variable extrathoracic lesion (such as vocal cord dysfunction) selectively flattens the inspiratory limb, while a variable intrathoracic lesion flattens the expiratory limb.9PubMed. Flow volume loops in the evaluation of upper airway obstruction These patterns can flag problems that have nothing to do with the lungs themselves.
Quality Grades and Why You Should Check Them
This is the part of the report most people skip, but quality indicators are worth a glance because a technically poor test can produce misleading numbers. The 2019 ATS/ERS standards assign each maneuver a quality grade, and some reports display these grades alongside each attempt.10PubMed Central. Standardization of Spirometry 2019 Update Common artifacts include coughing during the first second (which inflates or distorts the FEV1), an early termination where the patient stops blowing before reaching a plateau, a slow start that results in excessive back-extrapolated volume, and glottic closure where the throat involuntarily closes mid-breath.11National Institute for Occupational Safety and Health. Spirometry Quality Assurance: Common Errors and Their Impact on Test Results
Repeatability is another quality check. The report typically shows results from several attempts. For adults, the two best FVC values should be within 150 mL of each other, and the same goes for FEV1.10PubMed Central. Standardization of Spirometry 2019 Update If the difference is larger, it suggests the patient could not reproduce maximal effort consistently, and the results need to be treated with more caution. The reported “best” values are selected from the acceptable maneuvers, often by taking the largest FVC from one attempt and the largest FEV1 from any acceptable attempt, even if they come from different blows.
FEF 25–75% and Other Secondary Numbers
Below the main values, you might see FEF 25–75%, which measures airflow speed in the middle portion of the breath. It reflects what is happening in the smaller airways and tends to drop before FEV1 does in early disease. In one longitudinal study, people with normal FEV1 and FVC but low FEF 25–75% had a roughly three-fold higher risk of developing COPD over the following decade compared to those with normal FEF 25–75%, even after adjusting for smoking.12PubMed Central. FEF 25-75% Values in Patients with Normal Lung Function Can Predict the Development of Chronic Obstructive Pulmonary Disease In symptomatic smokers without overt airflow limitation, low FEF 25–75% was associated with lower FEV1 and FEV1/FVC values even when smoking history was accounted for, suggesting it picks up early small-airway disease that the main numbers have not yet flagged.13BMJ Open Respiratory Research. Small airway function measured using forced expiratory flow between 25% and 75% of vital capacity and its relationship to airflow limitation in symptomatic ever-smokers
That said, FEF 25–75% is more variable from blow to blow than FEV1 or FVC, and many guidelines do not use it as a primary diagnostic criterion. Think of it as an early-warning signal rather than a definitive finding. If your FEV1 and ratio are normal but your FEF 25–75% is low, it is worth discussing with your doctor, especially if you have symptoms or exposure risks.
When Spirometry Points to Additional Tests
Spirometry tells you about airflow and the volume of a forced breath, but the lungs do more than move air. When the spirometry picture is ambiguous or incomplete, clinicians often order complementary tests. Full lung volume measurement (plethysmography) is needed whenever restriction is suspected, as discussed earlier. The diffusing capacity test (DLCO) measures how efficiently gas transfers from your lungs into your blood, and it adds a different dimension to interpretation. In a smoker with confirmed obstruction, a low DLCO increases the probability that emphysema is the primary problem, while a normal DLCO points more toward chronic asthma. In someone with a spirometric restrictive pattern, a low DLCO raises concern for interstitial lung disease, while a normal DLCO makes a chest-wall or neuromuscular cause more likely.14PubMed. Office-based DLCO tests help pulmonologists to make important clinical decisions And in the somewhat puzzling situation of normal spirometry with unexplained shortness of breath, a low DLCO can raise the possibility of pulmonary vascular disease.
Tracking Changes Over Time
A single spirometry report is a snapshot. Serial reports tell a story. Everyone loses a small amount of lung function each year as a normal part of aging, typically around 20–30 mL of FEV1 per year in healthy adults. The clinical question is whether you are losing more than expected. In occupational health settings, both the American Thoracic Society and the American College of Occupational and Environmental Medicine flag an annual decline of 15% or more in FEV1 as excessive and warranting investigation.15PubMed Central. Limits of longitudinal decline for the interpretation of annual changes in FEV1 in individuals
Workers exposed to dust, vapors, gases, and fumes show an accelerated decline. A meta-analysis of population-based studies found that people ever exposed to such workplace hazards lost an extra 1.8 to 3.3 mL of FEV1 per year compared to unexposed individuals.16Occupational and Environmental Medicine. Ever and cumulative occupational exposure and lung function decline in longitudinal population-based studies That sounds small in a single year, but over a 30-year career it compounds into a clinically meaningful loss. This is why workplace spirometry programs test annually and compare each result against your personal baseline, not just against a population predicted value. If your report includes trend data, look at the direction and steepness of the decline rather than obsessing over any single number.
Spirometry in Children
Children can perform spirometry from about age three, but the technique demands cooperation that young kids sometimes struggle with. Shorter expiration times are expected and acceptable because children’s lungs have higher elastic recoil and empty faster. The quality thresholds are adjusted accordingly: for children six and under, repeatability criteria are loosened to 100 mL or 10% of the best effort, whichever is greater.10PubMed Central. Standardization of Spirometry 2019 Update In very young children (ages three to five), studies have used even more generous thresholds and found that the vast majority can still produce acceptable and repeatable results with proper coaching.17PubMed. Spirometry in 3-5-year-old children with asthma
FEV0.5 and FEV0.75 (the volume expired in the first half-second and three-quarters of a second) sometimes appear on pediatric reports because young children finish blowing so fast that FEV1 may capture nearly all of their FVC, making it less useful as a standalone number. Pediatric reference equations also differ from adult ones, and the GLI 2012 equations cover ages three and up, providing continuity into adulthood. If your child’s report looks confusing, the most important thing to check is still the same: the FEV1/FVC ratio relative to the LLN, and whether the quality grades indicate the child cooperated well enough for the numbers to be trusted.
Contraindications and Safety Flags
Spirometry is generally very safe, but the forced exhalation maneuver generates substantial pressure changes in the chest. The highest-risk contraindications involve recent cardiovascular events such as heart attack, pulmonary embolism, or an ascending aortic aneurysm. Recovery from major thoracic, abdominal, or head surgery also poses risk, with the level depending on how recent and how extensive the procedure was.18Postgraduate Medical Journal. An update on contraindications for lung function testing If you have been scheduled for spirometry soon after a surgery or cardiovascular event and the referring clinician is not your surgeon or cardiologist, it is worth confirming that the timing is appropriate. For routine screening, these situations rarely apply, but the technician performing the test should always review your history beforehand.