Guide — Pulmonary Function Testing
Interpreting Pulmonary Function Tests
A systematic way to read a PFT - start with the FEV₁/FVC ratio to find obstruction, use the FVC and TLC to find restriction, grade severity by percent predicted, check bronchodilator response, and let DLCO narrow the cause. Read the same values in the same order every time and the report stops being a wall of numbers.
10 min read · Pulmonary Function Testing
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
A pulmonary function test is read in a fixed order, the same way every time. The ratio finds obstruction, the volumes find restriction, percent predicted grades severity, and reversibility and DLCO point to the cause. Real reports are interpreted with reference equations and the clinical picture — not against a single textbook number.
Key Concepts — The Interpretation Algorithm
- FEV₁/FVC ratio.A low ratio (below 0.70 or below the lower limit of normal) means OBSTRUCTION. A normal or high ratio means no obstruction — look for restriction instead.
- Grade or pivot. If obstructed, grade severity by the FEV₁ percent predicted. If the ratio is normal but the FVC is low, suspect RESTRICTION and confirm it with the TLC.
- Confirm restriction with TLC. Restriction is confirmed by a reduced TLC, not by spirometry alone. A low TLC with a normal or high ratio is restriction.
- Watch for a mixed pattern. A low ratio AND a low TLC means combined obstruction and restriction.
- Bronchodilator response.A significant increase — at least 12% and at least 200 mL in FEV₁ or FVC — supports reversible obstruction (asthma).
- DLCO narrows the cause. Obstruction with a low DLCO suggests emphysema; restriction with a low DLCO suggests interstitial disease.
Assessment & Findings — Grading Severity
Once obstruction is established, severity is graded off the FEV₁ as a percent of predicted (ATS/ERS 2005):
| Severity | FEV₁ % Predicted |
|---|---|
| Mild | 70% or above |
| Moderate | 60 – 69% |
| Moderately severe | 50 – 59% |
| Severe | 35 – 49% |
| Very severe | below 35% |
COPD uses its own FEV₁ percent-predicted grades from GOLD:
| GOLD Grade | FEV₁ % Predicted |
|---|---|
| GOLD 1 (mild) | 80% or above |
| GOLD 2 (moderate) | 50 – 79% |
| GOLD 3 (severe) | 30 – 49% |
| GOLD 4 (very severe) | below 30% |
RT Priorities & Interventions
- Read in order. Ratio, then volumes (TLC), then percent-predicted severity, then reversibility, then DLCO. The sequence is what keeps you from misnaming a pattern.
- Use the patient’s predicted values. Compare to predicted values built from age, sex, height, and ethnicity — not a single textbook number.
- Flag what changes management. Significant reversibility points to asthma; a low DLCO with obstruction points to emphysema. Those are the findings the team acts on.
Common Pitfalls
- Calling a low FVC “restriction” without a TLC — it may be a poor effort or air trapping from obstruction.
- Grading severity off the FEV₁ alone without confirming the obstruction with the ratio.
- Ignoring the predicted-value reference set and reading absolute numbers.
Board Exam Pearls
- Ratio first: a low ratio is obstruction; restriction requires a low TLC.
- Severity is graded by FEV₁ percent predicted.
- A significant bronchodilator response is at least 12% AND at least 200 mL improvement.
- Mixed pattern = a low ratio plus a low TLC.
- Match the pattern to the DLCO to name the disease.
FAQ
What is the first value I should look at on a PFT?
The FEV₁/FVC ratio. A low ratio establishes obstruction. If the ratio is normal but the FVC is low, you suspect restriction and confirm it with a total lung capacity - spirometry alone cannot.
How is severity graded?
By the FEV₁ as a percent of predicted: roughly mild at 70% or above, moderate 60-69%, moderately severe 50-59%, severe 35-49%, and very severe below 35% (ATS/ERS). COPD has its own GOLD grades by FEV₁ percent predicted.
What is a mixed pattern?
Both obstruction and restriction together - a low FEV₁/FVC ratio (obstruction) plus a reduced total lung capacity (restriction). It needs lung volumes to identify, because the low FVC of obstruction alone can mimic restriction.
How does bronchodilator response fit in?
After the baseline test, a repeat after a bronchodilator that improves FEV₁ or FVC by at least 12% and 200 mL is a significant response, supporting reversible obstruction such as asthma. A negative result does not exclude asthma.
Put it to work
Put the reversibility step into practice: enter the pre and post FEV₁ and see whether it meets the 12% and 200 mL criterion.
Open the Bronchodilator Response calculator →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Pulmonary function test interpretation chapters.
- Pellegrino R, Viegi G, Brusasco V, et al. Interpretative strategies for lung function tests. Eur Respir J. 2005;26(5):948-968.