How should I interpret pulmonary function test results?

How to Interpret Pulmonary Function Tests

Interpretation of PFTs follows a systematic three-step approach: first assess test quality, then compare results to reference values using the 5th percentile (or Z-scores) as the lower limit of normal, and finally classify the pattern as obstructive, restrictive, mixed, or normal based on FEV1/FVC ratio and TLC measurements 1.

Step 1: Assess Test Quality First

Before interpreting any numerical values, you must evaluate test quality—this is the most commonly overlooked step that leads to misinterpretation 1, 2. Poor quality tests may still contain useful information, but you need to identify and document the limitations. Never rely solely on computer-generated interpretations without reviewing the raw data and flow-volume curves 1.

Key quality indicators to examine:

• Flow-volume loop configuration and reproducibility
• Patient effort (maximal inspiration and complete exhalation)
• Technical acceptability per ATS/ERS standards

Step 2: Compare to Reference Values

Use appropriate reference equations matched to the patient's:

• Age, sex, height (measured with stadiometer, not stated height)
• Ethnicity when relevant
• Geographic population 1

Values below the 5th percentile of predicted are considered abnormal 1. In the US, NHANES III equations (ages 8-80) are recommended for spirometry; in Europe, ECCS 1993 equations are commonly used, though no single set is universally recommended 1.

Step 3: Pattern Classification

Obstructive Pattern

• FEV1/FVC ratio < 5th percentile (or < 0.70 in adults) 1, 3
• TLC normal or increased
• Flows reduced across entire volume range
• Suggests: COPD, asthma, bronchiectasis

Critical pitfall: When both FEV1 and FVC are decreased but FEV1/FVC is normal, this usually indicates:

1. Incomplete inhalation/exhalation (poor effort)
2. Extremely slow flow preventing complete exhalation
3. Patchy small airway collapse 1

In this scenario, measure TLC to differentiate—if TLC is normal, it's obstruction (often with air trapping); if TLC is low, it's restriction 1. Consider bronchodilator testing if maximal effort was given 1.

Restrictive Pattern

• TLC < 5th percentile of predicted 1
• FEV1/FVC ratio normal or increased (>85-90%)
• Flow-volume curve shows convex pattern

Major pitfall: A reduced VC alone does NOT prove restriction—it's associated with low TLC only about 50% of the time 1. You must measure TLC by body plethysmography to confirm restriction. Never use single-breath VA from DLCO testing to diagnose restriction, as it systematically underestimates TLC, especially in obstruction (by up to 3L in severe cases) 1.

Mixed Pattern

• Both FEV1/FVC < 5th percentile AND TLC < 5th percentile 1
• Cannot be diagnosed from spirometry alone
• Requires formal lung volume measurement

Normal Pattern

• All values ≥ 5th percentile of predicted
• FEV1/FVC ratio preserved
• Flow-volume curve configuration normal

Additional Interpretive Points

Bronchodilator Response

Test for reversible airflow obstruction when obstruction is present. Significant response = FEV1 or FVC improvement >12% AND >200mL in adults (>12% alone in children 5-18 years) 3. This distinguishes reversible conditions like asthma from fixed obstruction like COPD.

Lung Volume Measurements

While not mandatory for diagnosing obstruction, lung volumes help characterize disease:

• Increased TLC, RV, or RV/TLC ratio suggests emphysema, asthma, or hyperinflation 1
• Use body plethysmography, not gas dilution, in obstructive disease
• Pneumothorax/bullae show normal plethysmographic TLC but low gas dilution TLC 1

Severity Grading

Based on FEV1 percentage of predicted once pattern is established 3.

Common Pitfalls to Avoid

1. Diagnosing restriction from reduced VC alone—requires TLC confirmation 1
2. Using fixed FEV1/FVC ratio of 0.70 in all ages—consider 5th percentile for accuracy 1
3. Ignoring flow-volume curve morphology—provides critical clues about effort and pathology 1
4. Accepting computer interpretation without quality review 1
5. Using single-breath VA to assess restriction in obstructive patients 1

The interpretation must always integrate PFT patterns with clinical context, symptoms, and imaging to arrive at the correct diagnosis 1.