Pulmonary Function Test Protocol and Interpretation
A complete PFT should include spirometry (FVC, FEV1, FEV1/FVC), lung volumes by plethysmography (TLC, RV), and diffusing capacity (DLCO), with interpretation based on comparing results to the 5th percentile (lower limit of normal) of predicted values using appropriate reference equations, followed by systematic pattern recognition to identify obstruction, restriction, or mixed defects 1, 2.
Test Performance Protocol
Essential Components to Measure
Spirometry:
- FVC (forced vital capacity)
- FEV1 (forced expiratory volume in 1 second)
- FEV1/FVC ratio
- Slow vital capacity (VC) - particularly useful when obstruction is suspected 1
Lung Volumes (by body plethysmography preferred):
- TLC (total lung capacity)
- RV (residual volume)
- RV/TLC ratio 2
Diffusing Capacity:
- DLCO (diffusing capacity for carbon monoxide)
- VA (alveolar volume)
- KCO (transfer coefficient) 2
Critical Quality Control Steps
Before interpreting any numerical results, you must assess test quality - this is the most common error in PFT interpretation, particularly when relying on computer-generated reports 2. Document the quality of effort, reproducibility of maneuvers, and any technical limitations that could affect accuracy.
Interpretation Algorithm
Step 1: Compare to Reference Values
Use the 5th percentile as the lower limit of normal (LLN) rather than fixed percentages like 80% predicted 2. The 5th percentile approach prevents overdiagnosis in older patients and underdiagnosis in younger patients. Reference equations should match the patient's:
- Age
- Sex
- Height
- Ethnicity 2
Display results with measured values first, then predicted values, and ideally include z-scores to show standard deviation units from predicted 1.
Step 2: Identify Ventilatory Pattern
Obstruction:
- FEV1/FVC < 5th percentile of predicted 2
- Report FEV1/FVC as percentage of predicted ratio to minimize miscommunication 1
- Lung volumes may show increased TLC, RV, or RV/TLC suggesting hyperinflation 2
Restriction:
- TLC < 5th percentile of predicted 2
- FEV1/FVC normal or increased (>85-90%)
- Critical pitfall: A reduced VC alone does NOT prove restriction - it's associated with low TLC only about 50% of the time 2. You must measure TLC to confirm restriction.
- Never use single-breath VA from DLCO testing to diagnose restriction - it systematically underestimates TLC, especially in obstruction (by up to 3L in severe cases) 2
Mixed Defect:
- Both FEV1/FVC AND TLC < 5th percentile 2
Important Pattern Recognition: When FEV1 and FVC are both reduced but FEV1/FVC is normal, this most commonly indicates poor effort (incomplete inhalation/exhalation). However, it can rarely represent true obstruction with severe air trapping. Check the flow-volume curve for concavity and measure TLC - if TLC is normal and the curve shows late expiratory slowing, this suggests patchy small airway collapse 2. Repeat after bronchodilator to assess reversibility.
Step 3: Assess DLCO
Abnormal if < 5th percentile of predicted 2. DLCO provides information about:
- Alveolar-capillary membrane integrity
- Capillary blood volume
- Ventilation-perfusion matching
Ensure predicted values for DLCO, VA, and KCO come from the same reference source and document adjustments for hemoglobin, carboxyhemoglobin, and altitude 2.
Step 4: Grade Severity
Once a pattern is identified, grade the severity based on how far below the LLN the key parameters fall (FEV1 for obstruction, TLC for restriction, DLCO for gas exchange abnormality).
Step 5: Compare to Prior Studies
If previous PFTs exist, document clinically significant changes. This "comparison with self" is crucial for monitoring disease progression or treatment response 2.
Common Pitfalls to Avoid
Do not over-report spirometric parameters - only FVC, FEV1, and FEV1/FVC need routine reporting. Adding multiple flow measurements (FEF25-75, FEF75, etc.) increases false-positive rates without adding clinical utility 1. In healthy populations, examining 14 spirometric parameters yields 24% "abnormal" results versus 10% when examining only the three key values 2.
Mid-expiratory flow reductions are not specific for small airway disease in individual patients, despite being commonly reported this way 2.
Beware borderline values - patients near the 5th percentile threshold have the highest risk of misclassification 2. In these cases, consider additional testing (bronchodilator response, exercise testing, respiratory muscle strength) based on clinical context.
Pneumothorax and non-communicating bullae create a special pattern: normal FEV1/FVC and normal TLC by plethysmography, but low FEV1 and VC. Gas dilution techniques will show falsely low TLC in these conditions 2.
Reporting Standards
The report should 1:
- Comment on test quality first
- Display measured values before predicted values
- Show normal limits for each parameter
- Include z-scores when possible
- Address the clinical question that prompted testing
- Avoid merely stating which values are "normal" or "low" - provide physiologic interpretation
The most meaningful interpretation addresses the specific clinical question - whenever possible, obtain information about symptoms (cough, dyspnea, wheezing), smoking status, chest radiograph findings, recent hemoglobin, and suspected diagnoses before interpreting 2.