How do you interpret Pulmonary Function Tests (PFTs)?

How to Interpret Pulmonary Function Tests

Interpret PFTs using a systematic, stepwise algorithm: first assess test quality, then compare results to reference values, identify physiological patterns (obstruction, restriction, or mixed), grade severity, and finally answer the specific clinical question that prompted the testing. 1, 2

Step 1: Assess Test Quality First

• Always review technical quality before relying on numerical results—this is the most common mistake in PFT interpretation, particularly when depending on computer-generated reports 1, 2
• Verify that spirometry meets acceptability and reproducibility criteria per ATS/ERS standards 1
• Document any technical limitations that may affect interpretation accuracy, as suboptimal tests may still contain useful information but require acknowledgment of potential errors 1

Step 2: Compare to Reference Values

• Use predicted values based on the patient's age, sex, height, and ethnicity from equations derived from healthy reference populations 1, 2
• Define abnormal as below the 5th percentile (lower limit of normal) rather than the traditional <80% predicted, as recommended by current ERS/ATS guidelines 1
• Measure height and weight at each visit, as these anthropometric variables directly affect predicted values 1

Race and Ethnicity Considerations

• Statistically significant differences exist in FEV1 and FVC (but not FEV1/FVC ratio) between racial groups, with people of White European ancestry having larger values on average compared to those of African or Asian ancestry 1
• These differences are less clinically significant in clearly normal or abnormal results but may impact interpretation in borderline cases 2
• Adjusting for body proportions and socioeconomic factors attenuates but does not eliminate observed differences between groups 1

Step 3: Identify Physiological Patterns

Obstructive Pattern

• Defined by FEV1/FVC ratio <0.70 (or below 5th percentile), indicating disproportionate reduction in maximal airflow relative to lung volume 1, 2
• Look for concave shape on the flow-volume curve, which reflects slowing of expiratory flow 1
• Early small airway disease may show terminal spirogram slowing even when FEV1 is preserved 1

Restrictive Pattern

• Requires measurement of total lung capacity (TLC) <80% predicted—spirometry alone showing reduced FVC with normal FEV1/FVC is insufficient to diagnose restriction 1, 2
• Common pitfall: failing to measure lung volumes when restrictive pattern is suspected based on spirometry alone 2
• VC may be only slightly impaired in diffuse interstitial diseases despite marked loss of diffusing capacity 2

Mixed Pattern

• Presence of both reduced FEV1/FVC ratio and reduced TLC 2

Step 4: Grade Severity

Base severity primarily on FEV1 % predicted for obstructive, restrictive, and mixed defects (not on FEV1/FVC ratio): 2

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

DLCO Severity

• DLCO <60% predicted is associated with significantly higher mortality (25%) and pulmonary morbidity (40%), particularly in preoperative lung resection evaluation 2
• Use 5th percentile as lower limit of normal 2
• Always adjust DLCO for hemoglobin and carboxyhemoglobin levels, especially when monitoring for drug toxicity 2

Step 5: Assess Bronchodilator Response (if performed)

• Evaluate change in FEV1 and FVC after bronchodilator administration 1, 2
• Significant response typically defined as ≥12% and ≥200 mL increase in FEV1 or FVC 1

Step 6: Compare with Previous Results

• Evaluate for clinically significant changes over time by comparing current results with prior tests—this is the primary reason for 60% of PFTs performed 2, 3
• Maintain consistent interpretation approach within your laboratory to avoid falsely attributing changes to disease progression when they reflect interpretation variability 2

Step 7: Answer the Clinical Question

• Address the specific indication for testing: diagnosis, disease monitoring, preoperative evaluation, disability assessment, or treatment response 1, 2
• In COPD, FEV1 correlates with symptom severity and prognosis 2
• For lung cancer resection candidates, systematically measure both FEV1 and DLCO, as DLCO <60% indicates high surgical risk 2

Critical Pitfalls to Avoid

• Never rely solely on computer interpretations without quality review 1, 2
• Do not use FEV1/FVC ratio to determine severity of obstruction—use FEV1 % predicted instead 2
• Do not diagnose restriction without measuring lung volumes 2
• Recognize that upper airway obstruction may be life-threatening despite being classified as only "mildly reduced" by FEV1 % predicted 2
• PFTs are only one diagnostic tool and must be interpreted in full clinical context including symptoms, physical examination, chest radiograph, and relevant laboratory values 1, 2
• In borderline cases near thresholds of abnormality, patients are at greatest risk of misclassification—consider additional testing or longitudinal follow-up 1