How to Interpret Pulmonary Function Tests
Pulmonary function test interpretation follows a systematic, stepwise approach: first assess test quality, then compare results to reference values using the 5th percentile as the lower limit of normal, identify ventilatory patterns (obstructive, restrictive, or mixed), grade severity, and finally integrate findings with clinical context to answer the specific clinical question. 1
Step 1: Assess Test Quality First
Before interpreting any numerical values, always review and comment on test quality 1. This is a critical step that is commonly overlooked, especially when relying on computer-generated interpretations. Poor quality tests may still contain useful information, but you must identify the direction and magnitude of potential errors. Omitting quality review and relying solely on numerical results is a common mistake that leads to misinterpretation 1.
Step 2: Compare to Reference Values
Use the 5th percentile of predicted values as the lower limit of normal (LLN), not fixed ratios 1. Reference values must match the patient's:
- Sex
- Age
- Height
- Ethnicity
Critical pitfall: Using a fixed FEV1/FVC ratio of 0.7 leads to overdiagnosis of obstruction in older patients and underdiagnosis in younger patients 1. The 5th percentile approach avoids this systematic error.
Step 3: Identify Ventilatory Patterns
Focus interpretation on four core parameters: FEV1, VC (vital capacity), FEV1/VC ratio, and TLC (total lung capacity) 1. Use the largest available VC whether obtained by inspiration (IVC), slow expiration (SVC), or forced expiration (FVC) 1.
Obstructive Pattern
- FEV1/VC < 5th percentile of predicted 1
- This single criterion defines obstruction
- Lung volume measurements are not mandatory for diagnosis but help identify emphysema, asthma, or hyperinflation 1
Common pitfall: When both FEV1 and FVC decrease together with a normal FEV1/FVC ratio, this usually reflects poor effort or incomplete exhalation, NOT obstruction 1. The flow-volume curve should show concavity at end-expiration if true obstruction exists. Consider repeating after bronchodilator to clarify 1.
Restrictive Pattern
- TLC < 5th percentile of predicted AND normal FEV1/VC 1
- A reduced VC alone does NOT prove restriction—it's associated with low TLC only about 50% of the time 1
- Suspect restriction when VC is reduced, FEV1/VC is increased (>85-90%), and the flow-volume curve shows a convex pattern 1
Critical pitfall: Never use VA from a single-breath DLCO test to diagnose restriction—it systematically underestimates TLC, especially in obstruction where it can be off by up to 3 liters 1. This leads to massive misclassification errors.
Mixed Pattern
- Both FEV1/VC AND TLC < 5th percentile of predicted 1
- Cannot be diagnosed from spirometry alone
- If FEV1/VC is low and VC is normal, you can rule OUT superimposed restriction 1
- If FEV1/VC is low and VC is reduced without TLC measurement, state that restriction cannot be excluded 1
Step 4: Grade Severity
Use FEV1 % predicted to classify severity 1:
| Severity | FEV1 % Predicted |
|---|---|
| Mild | >70% |
| Moderate | 60-69% |
| Moderately Severe | 50-59% |
| Severe | 35-49% |
| Very Severe | <35% |
Important limitation: This severity grading has poor applicability to upper airway obstruction (like tracheal stenosis), where life-threatening obstruction may be classified as "mild" 1. FEV1 correlates with mortality and morbidity in COPD but cannot accurately predict symptoms or prognosis for individual patients 1.
DLCO is also an important mortality predictor in the general population and post-lung resection patients 1.
Step 5: Avoid Over-Interpretation
Limit primary interpretation to FEV1, VC, FEV1/VC, and TLC 1. Examining multiple additional parameters (like MEF25-75%, FEF75) dramatically increases false-positive rates—from 10% with three parameters to 24% with 14 parameters in healthy subjects 1.
Only consider additional flow measurements after establishing the presence and severity of obstruction using the basic values. When FEV1 and FEV1/VC are normal, late expiratory flow abnormalities have limited clinical significance 1.
Step 6: Integrate Clinical Context
The final step is answering the specific clinical question that prompted the test 1. Be conservative in suggesting specific diagnoses based solely on PFT patterns—functional abnormalities indicate physiologic impairment but don't definitively identify underlying disease 1.
When values fall near the LLN, consider additional testing if clinically indicated: bronchodilator response, DLCO, gas exchange evaluation, respiratory muscle strength, or exercise testing 1.
Special consideration: When TLC is at the LLN in a patient with expected restriction (like post-lung resection), the restrictive defect may be difficult to prove if TLC remains above the 5th percentile 1.