How do you interpret Pulmonary Function Tests (PFTs) results?

How to Interpret Pulmonary Function Tests

Interpret PFTs using 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 the physiological pattern (obstructive, restrictive, or mixed), grade severity based on FEV1 % predicted, and finally integrate findings with clinical context to answer the specific diagnostic question. 1, 2

Step 1: Assess Test Quality First

Before interpreting any numerical values, verify that the test meets acceptability and reproducibility criteria 1:

• For spirometry: Use standardized quality grading (A through F) separately for FVC and FEV1, with grades A, B, or C considered usable 1
• For DLCO: Grade A requires VI/VC >90%, breathhold time 8-12 seconds, and sample collection <4 seconds 1
• Review the flow-volume curves directly rather than relying solely on computer interpretations 2
• Document any technical limitations that may affect interpretation 1

Common pitfall: Relying on computer-generated interpretations without reviewing actual test quality can lead to misdiagnosis 2

Step 2: Compare to Reference Values

Use appropriate reference equations matched to the patient's age, sex, height, and ethnicity 1:

• For spirometry: Use GLI-2012 (Global Lung Initiative) equations, which are valid from ages 3-95 years and include multiple ethnicities 1
• For DLCO: Use the new GLI diffusing capacity equations (published 2017) 1
• Define abnormal as below the 5th percentile (lower limit of normal), not fixed cutoffs like 80% predicted or FEV1/FVC <0.70 1

Critical distinction: The 5th percentile varies by age and height, preventing overdiagnosis in elderly patients and underdiagnosis in younger patients that occurs with fixed cutoffs 1

Step 3: Identify the Physiological Pattern

Obstructive Pattern

• Defined by: FEV1/VC ratio below the 5th percentile of predicted 1
• Use the largest available VC (whether inspiratory, expiratory, or slow) rather than just FVC 1
• Look for concave shape on the flow-volume curve indicating airflow limitation 1
• Does NOT require lung volume measurement to diagnose obstruction 1

Important caveat: When both FEV1 and FVC are reduced with normal FEV1/FVC ratio, this usually reflects poor effort or incomplete exhalation, not true obstruction—confirm with lung volume measurement showing normal TLC 1

Restrictive Pattern

• Defined by: TLC below the 5th percentile of predicted with normal FEV1/VC ratio 1
• Cannot be diagnosed by spirometry alone—requires direct measurement of TLC by plethysmography or dilutional methods 1
• Reduced VC with increased FEV1/VC (>85-90%) and convex flow-volume curve only suggests restriction 1

Critical error to avoid: Calling a pattern "restrictive" based only on reduced VC without measuring TLC leads to frequent misdiagnosis 1

Mixed Pattern

• Defined by: Both FEV1/VC and TLC below the 5th percentile of predicted 1

Step 4: Grade Severity

For obstructive, restrictive, and mixed defects, severity is based on FEV1 % predicted 2:

• Mild: >70% predicted
• Moderate: 60-69% predicted
• Moderately severe: 50-59% predicted
• Severe: 35-49% predicted
• Very severe: <35% predicted

For DLCO: Values <60% predicted are associated with significantly higher mortality (25%) and pulmonary morbidity (40%) in surgical candidates 2

Do NOT use the FEV1/VC ratio to grade severity—this is a common error 2

Step 5: Assess Additional Parameters

Hyperinflation

• Elevated TLC, RV, or RV/TLC ratio above the upper limit of normal suggests emphysema, asthma, or air trapping 1

Diffusing Capacity (DLCO)

• Adjust for hemoglobin and carboxyhemoglobin, especially when monitoring for drug toxicity 2
• Low DLCO with normal spirometry and lung volumes suggests early interstitial disease or pulmonary vascular disease 2
• VC may be only slightly impaired in diffuse interstitial diseases despite markedly reduced DLCO 2

Bronchodilator Response

• Evaluate if performed, noting significant improvement in FEV1 or FVC suggesting reversible airflow obstruction 1

Step 6: Integrate Clinical Context

PFT interpretations should be clear, concise, and address the specific clinical question 1:

• Record respiratory symptoms (cough, phlegm, wheezing, dyspnea), smoking status, and recent bronchodilator use 1
• Consider chest radiograph findings, hemoglobin value, and suspicion of neuromuscular disease or upper airway obstruction 1
• Avoid making specific disease diagnoses based solely on PFT patterns—use them to guide further evaluation 1

Essential principle: A mere statement of which values are normal or low is not helpful; interpretation must estimate post-test probability of disease considering pre-test probability and clinical context 1

Common Pitfalls to Avoid

• Using fixed cutoffs (80% predicted, FEV1/FVC <0.70) instead of the 5th percentile LLN 1, 2
• Diagnosing restriction without measuring TLC 1, 2
• Not adjusting DLCO for hemoglobin and carboxyhemoglobin 2
• Overlooking that upper airway obstruction may be life-threatening despite only mildly reduced FEV1 2
• Failing to review flow-volume curves and relying only on numerical values 2
• Using FEV1/VC ratio to determine severity (use FEV1 % predicted instead) 2

Assessing Change Over Time

When comparing serial PFTs, use the coefficient of repeatability rather than arbitrary percentage changes 1:

• Within a day: FEV1 changes <5%, DLCO changes >7% are significant 1
• Week to week: FEV1 changes >12%, DLCO changes >6 units are significant 1
• Year to year: FEV1 changes >15%, DLCO changes >10% are significant 1

Changes exceeding these thresholds likely represent true physiological change rather than measurement variability 1