Nursing care
Pulmonary Function Tests: reading the number and acting on it
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated September 2026
Short answer
Pulmonary function tests measure lung volumes and airflow, and interpretation centers on the FEV1/FVC ratio. A ratio below 70 per cent indicates obstruction, while a low FVC with a preserved ratio points to restriction. Bronchodilators are held before testing so baseline airway function is captured accurately.
What the test measures
Pulmonary function tests, most often performed by spirometry, measure how much air a patient can move and how fast they can move it. The two core values are forced vital capacity, the total volume exhaled after a maximal inhale, and forced expiratory volume in one second, the volume exhaled in the first second of that breath.
The relationship between these two numbers, expressed as FEV1/FVC, is what separates obstructive disease from restrictive disease. Spirometry can be supplemented with diffusion capacity testing (DLCO), which measures how well gas crosses the alveolar-capillary membrane, useful when a restrictive pattern needs further characterization, as in interstitial lung disease versus a chest wall limitation.
Normal ranges and what moves them
A normal FEV1/FVC ratio sits at or above 70 percent in most adults, though reference values are adjusted for age, height, sex, and ethnicity, and results are usually reported as a percentage of predicted normal. FVC itself is typically expected to fall between 80 and 120 percent of predicted.
Values shift with more than just disease. Poor patient effort, incomplete exhalation, a leaking mouthpiece seal, or recent bronchodilator use can all skew results. This is why bronchodilators, both short and long acting, are withheld for a defined period before testing, so the baseline reflects the patient's unmedicated airway status and any post-bronchodilator improvement can be measured accurately.
What a high result means
A pulmonary function test rarely produces a value that reads as abnormally 'high' in the way a lab value like potassium does. What clinicians look for instead is a normal or elevated FEV1/FVC ratio alongside reduced volumes, which can suggest a restrictive pattern rather than obstruction.
An elevated ratio with normal volumes is simply a normal study and should be interpreted in that light rather than treated as a finding requiring intervention. Where volumes are unexpectedly large relative to predicted values, technical error, such as poor calibration or an inaccurate height entry, should be considered before assuming supranormal lung function.
What a low result means
An FEV1/FVC ratio below 70 percent signals obstruction, seen in COPD and asthma, where airways narrow and trap air, so the patient exhales slowly relative to the total volume they can move. A low FVC with a preserved or even elevated ratio points instead to a restrictive pattern, where lung expansion itself is limited, as in pulmonary fibrosis, chest wall deformity, or neuromuscular weakness.
Severity within either pattern is graded by how far FEV1 falls below predicted, with mild, moderate, and severe categories guiding treatment intensity. A significant improvement in FEV1 after bronchodilator administration, generally 12 percent and 200 mL or more, supports a diagnosis of reversible airway disease such as asthma over fixed obstruction like COPD.
Nursing actions by result
For an obstructive pattern, anticipate bronchodilator therapy, inhaled corticosteroids if indicated, and patient teaching on inhaler technique, since poor technique is a common reason therapy underperforms. Monitor oxygen saturation and watch for signs of an acute exacerbation, including increased work of breathing and use of accessory muscles.
For a restrictive pattern, focus shifts to identifying the underlying cause, whether pulmonary, chest wall, or neuromuscular, since management differs substantially by category. Document baseline results clearly so serial testing can track disease progression or response to treatment over time, and flag any test with poor effort or technical error for repeat testing rather than letting an unreliable value guide care decisions.
Patient preparation and teaching
Instruct the patient to avoid smoking for at least one hour before testing and to avoid a heavy meal that could limit diaphragmatic excursion. Loose, non-restrictive clothing helps ensure a full inhale and exhale during the maneuver.
Confirm with the ordering provider which medications to hold and for how long, since short-acting bronchodilators are typically held for four to six hours and long-acting agents for 12 to 24 hours or longer, depending on the agent. Explain the test itself to the patient beforehand: a forceful, complete exhale into the mouthpiece, repeated several times to get consistent, reproducible results, since anxiety or confusion about the maneuver is a common cause of a poor-effort test.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our respiratory practice questions are the closest set to what this page covers.
Common questions
What FEV1/FVC ratio indicates obstruction?
A ratio below 70 percent indicates an obstructive pattern, as seen in COPD and asthma. The lower the ratio, the more significant the airflow obstruction, and severity is further staged by how far FEV1 falls below predicted.
Why are bronchodilators held before pulmonary function tests?
Holding bronchodilators captures the patient's true baseline airway function without medication effect. It also allows a meaningful post-bronchodilator comparison to assess reversibility, which helps distinguish asthma from fixed obstructive disease.
What does a low FVC with a normal ratio mean?
This pattern suggests restrictive lung disease rather than obstruction, since both FEV1 and FVC are reduced proportionally so the ratio stays normal or even rises. Causes include pulmonary fibrosis, chest wall abnormality, or neuromuscular weakness limiting lung expansion.
How is asthma distinguished from COPD on spirometry?
Both show an obstructive pattern with a reduced FEV1/FVC ratio, but asthma typically shows significant reversibility after bronchodilator administration, generally a 12 percent and 200 mL rise in FEV1, while COPD shows limited or no reversibility.
What should a nurse teach a patient before spirometry?
Avoid smoking and a heavy meal beforehand, wear loose clothing, and hold bronchodilators as directed by the ordering provider. Explain that the test requires several forceful, complete exhales to get a reliable result.