How should I interpret a pulmonary function test (PFT)?

How to Interpret Pulmonary Function Tests

Systematic Four-Step Interpretation Algorithm

Interpret PFTs using a systematic four-step approach: (1) assess technical quality first, (2) compare results to appropriate reference values using the 5th percentile as the lower limit of normal, (3) identify the physiological pattern (obstructive, restrictive, or mixed), and (4) answer the specific clinical question that prompted testing. 1

Step 1: Assess Technical Quality

• Review the raw flow-volume curves and verify acceptability/reproducibility criteria before trusting any numerical results—this is the single most common source of interpretation error. 1
• Examine the flow-volume loop visually for characteristic patterns: a concave shape indicates obstruction, while a convex pattern suggests restriction. 2
• Ensure the patient performed maximal inspiratory and expiratory efforts; concomitantly decreased FEV1 and FVC with a normal FEV1/FVC ratio most frequently reflects incomplete inhalation or exhalation rather than true disease. 3
• Document any technical limitations, as suboptimal tests may still provide useful information if you understand the direction and magnitude of potential errors. 1

Step 2: Compare to Reference Values

• Use predicted values matched to the patient's age, sex, measured height (not self-reported), and ethnicity from reference equations with similar equipment and population characteristics. 3, 1
• Apply the 5th percentile of the reference population as the lower limit of normal for all parameters—avoid fixed cutoffs like "80% predicted" which cause substantial misclassification, especially in adults. 1
• Ensure all spirometric indices (FVC, FEV1, FEV1/FVC) come from the same reference source to maintain internal consistency. 3, 1
• When race/ethnic-specific equations are unavailable, apply adjustment factors: 0.88 for Black patients (FEV1, FEV1/FVC, TLC), 0.93 for Black patients (FRC, RV), and 0.94 for Asian-American patients—but never apply race adjustments to the FEV1/FVC or FEV1/VC ratios. 1

Step 3: Identify the Physiological Pattern

Start with the FEV1/VC ratio (or FEV1/FVC if VC unavailable) as the primary parameter for pattern recognition. 3, 2

Obstructive Pattern

• FEV1/VC ratio below the 5th percentile indicates obstruction—this is the most important parameter for identifying obstructive impairment and predicts morbidity and mortality even when FEV1 is normal. 3, 2
• Do not use a fixed FEV1/FVC < 0.70 threshold, as it generates false-positive COPD diagnoses in men >40 years and women >50 years, particularly in elderly never-smokers. 1
• The flow-volume curve shows a characteristic concave shape with FEV1 proportionally more reduced than VC. 2
• Measure TLC to assess hyperinflation: increased TLC, RV, or RV/TLC ratio suggests emphysema, asthma, or other obstructive diseases. 3, 2
• Additional flow measurements (FEF 25-75%, MEF) should only be considered after establishing obstruction with FEV1/VC; when FEV1/VC is borderline, these may suggest early airway obstruction, especially in children with cystic fibrosis. 3

Restrictive Pattern

• A TLC below the 5th percentile with normal FEV1/VC confirms true restriction—reduced VC alone does not prove restriction, as only 50% of cases with low VC actually have low TLC. 3, 2
• Suspect restriction when VC is reduced, FEV1/VC is increased (>85-90%), and the flow-volume curve shows a convex pattern. 3, 2
• Never use single-breath VA from DLCO testing to diagnose restriction, as it systematically underestimates TLC by up to 3 L in severe obstruction, greatly increasing misclassification risk. 3
• In pneumothorax and noncommunicating bullae, TLC measured by body plethysmography will be normal despite low FEV1 and VC, while gas dilution techniques will show falsely low TLC. 3

Mixed Pattern

• Mixed defects are defined when both FEV1/VC and TLC are below the 5th percentile. 3
• When FEV1/VC is low and VC is reduced but TLC is not measured, state that VC is reduced (probably due to hyperinflation) but that a superimposed restrictive component cannot be excluded without TLC measurement. 3

Normal or Borderline Results

• When values lie close to the 5th percentile threshold, misclassification risk is greatest; consider repeat testing, lung volumes, or diffusion studies. 1
• A normal FEV1/VC with concomitantly decreased FEV1 and FVC may indicate poor effort, patchy peripheral airflow obstruction, or inability to exhale long enough due to very slow flow. 3

Step 4: Answer the Clinical Question

Assess Severity

• Grade severity based on FEV1 % predicted: Mild >70%, Moderate 60-69%, Moderately severe 50-59%, Severe 35-49%, Very severe <35%. 1
• Do not use FEV1/VC ratio to determine severity—only use it to identify the presence of obstruction. 1
• Recognize that FEV1 correlates poorly with symptoms in individual patients despite its prognostic value. 1, 2
• Upper airway obstruction may be life-threatening despite being classified as only mildly reduced by FEV1 % predicted. 1

Evaluate DLCO

• Low DLCO (<60% predicted) indicates parenchymal disease, emphysema, vascular involvement, and is an important predictor of mortality. 1, 2
• In preoperative lung cancer evaluation, DLCO <60% indicates 25% mortality risk and 40% pulmonary morbidity risk after lung resection. 1
• Always adjust DLCO for hemoglobin and carboxyhemoglobin, especially when monitoring for drug toxicity. 1, 2
• VC may be only slightly impaired in diffuse interstitial diseases despite markedly reduced DLCO. 1

Check Bronchodilator Response

• A significant bronchodilator response is defined as an increase in FEV1 or FVC of >12% AND >200 mL. 2
• Absence of acute bronchodilator response does not rule out clinical benefit from long-term bronchodilator therapy. 2
• Repeat spirometry after bronchodilator when you observe reduced FEV1 and FVC with normal FEV1/FVC in a patient with maximal effort—significant improvement suggests reversible airflow obstruction. 3

Common Pitfalls to Avoid

• Never rely solely on computer-generated interpretations without personally reviewing test quality and flow-volume curves. 1
• Do not use FEV1/VC ratio to grade severity of obstruction—use FEV1 % predicted instead. 1
• Avoid diagnosing restriction based on spirometry alone without measuring lung volumes. 1
• Do not fail to adjust DLCO for hemoglobin, especially when monitoring for drug toxicity. 1
• Recognize that examining multiple parameters simultaneously (>3-4 measurements) dramatically increases false-positive rates: 10% abnormality rate with 3 parameters increases to 24% with 14 parameters, even in healthy subjects. 3
• Remember that PFTs are only one diagnostic tool and must be interpreted within the full clinical context—they cannot by themselves distinguish among potential causes of abnormalities. 1, 4
• Ensure interpretation strategies remain consistent within your laboratory to avoid falsely suggesting disease progression when only the interpretation approach has changed. 3, 1