How do I interpret a pulmonary function test (PFT) study in an adult?

How to Interpret a Pulmonary Function Test (PFT)

Interpret PFTs using a systematic four-step algorithm: first assess test quality, then compare results to reference values using the 5th percentile as the lower limit of normal, identify physiological patterns (obstruction, restriction, or mixed), and finally answer the specific clinical question that prompted the test. 1

Step 1: Assess Test Quality First

• Always review technical quality before relying on numerical results—this is the most common mistake in PFT interpretation. 1
• Verify that the test meets acceptability and reproducibility criteria for spirometry, lung volumes, and diffusing capacity measurements. 2
• Tests that are suboptimal may still contain useful information, but you must identify the problems and document the direction and magnitude of potential errors. 1
• Never rely solely on computer-generated interpretations without reviewing the raw data and flow-volume curves. 2

Step 2: Compare to Appropriate Reference Values

• Use predicted values from reference equations that match the patient's age, sex, height, and ethnicity. 1
• Measure height with a stadiometer at the time of testing—never rely on stated height. 1
• Apply race/ethnic-specific reference equations when available; if unavailable, use adjustment factors (0.88 for Black patients for FEV1/FVC/TLC, 0.93 for FRC/RV; 0.94 for Asian Americans). 1
• Do not apply race adjustment factors to FEV1/FVC or FEV1/VC ratios. 1
• Ensure all parameters (FVC, FEV1, FEV1/FVC) come from the same reference source. 1

Step 3: Define Normal vs. Abnormal Using Proper Thresholds

• Use the 5th percentile of the reference population as the lower limit of normal—not fixed percentages like 80% predicted. 1
• Avoid using FEV1/FVC < 0.70 as a fixed cutoff, as this causes false-positive results in males >40 years and females >50 years, leading to overdiagnosis of COPD in elderly never-smokers. 1
• Using 80% predicted as the lower limit of normal is acceptable in children but leads to significant errors in adults. 1
• For variables that can be too high or too low (TLC, RV/TLC, DLCO), use both upper and lower limits. 1

Step 4: Identify Physiological Patterns

Obstruction

• Defined by FEV1/FVC below the lower limit of normal (5th percentile). 2
• Classify severity based on FEV1 % predicted: Mild >70%, Moderate 60-69%, Moderately severe 50-59%, Severe 35-49%, Very severe <35%. 2
• FEV1 correlates with symptom severity and prognosis in COPD. 2

Restriction

• Cannot be diagnosed by spirometry alone—requires measurement of total lung capacity (TLC). 3
• Defined by TLC <80% predicted (below 5th percentile). 2
• Spirometry showing reduced FVC with normal FEV1/FVC suggests restriction but requires lung volume confirmation to distinguish true restriction from pseudorestriction. 3

Gas Exchange Abnormality

• Assessed by diffusing capacity (DLCO) below the 5th percentile. 2
• DLCO <60% is associated with significantly higher mortality (25%) and pulmonary morbidity (40%) in patients undergoing lung resection. 2
• Adjust DLCO for hemoglobin and carboxyhemoglobin, especially when monitoring for drug toxicity. 2
• VC may be only slightly impaired in diffuse interstitial diseases despite marked loss of diffusing capacity. 2

Step 5: Answer the Clinical Question

• Integrate PFT results with the patient's clinical context, history, physical examination, and imaging findings. 4, 5
• Recognize that PFTs are only one tool and cannot by themselves distinguish among potential causes of abnormalities. 2, 4
• For preoperative evaluation of lung cancer patients, measure both FEV1 and DLCO systematically. 2

Critical Pitfalls to Avoid

• Do not use FEV1/VC ratio to determine severity of obstruction—use FEV1 % predicted instead. 2
• Do not fail to measure lung volumes when spirometry suggests a restrictive pattern. 2
• Do not overlook that upper airway obstruction may be life-threatening despite being classified as only mildly reduced by FEV1 % predicted. 2
• Maintain consistent interpretation approaches within your laboratory to avoid falsely inferring changes in patient condition when the change is actually due to different interpretation methods. 2
• In borderline cases near thresholds of abnormality, patients are at greatest risk of misclassification—consider additional testing. 1, 2