How do you interpret Pulmonary Function Test (PFT) results in a patient with a history of respiratory diseases such as asthma or Chronic Obstructive Pulmonary Disease (COPD)?

Interpreting Pulmonary Function Tests in Asthma and COPD

Interpret PFT results by first determining if obstruction exists (FEV1/FVC ratio below the lower limit of normal), then assess reversibility with bronchodilator testing (≥12% and ≥200 mL improvement in FEV1 indicates asthma), and grade severity using FEV1 percent predicted rather than the FEV1/FVC ratio. 1

Step 1: Establish the Presence of Obstruction

• Calculate the FEV1/FVC ratio first - this is the critical initial step to identify airflow obstruction, not FEV1 alone 1
• Use the lower limit of normal (LLN) defined as the 5th percentile rather than the fixed 70% cutoff when possible, as this accounts for age-related changes 1
• An FEV1/FVC ratio below the LLN confirms obstructive disease 1
• Never diagnose obstruction based on FEV1 alone - this is a common pitfall that leads to misdiagnosis 1

Step 2: Distinguish Asthma from COPD Using Reversibility Testing

Asthma Pattern

• Reversible obstruction: FEV1 improvement ≥12% AND ≥200 mL after bronchodilator administration confirms asthma 1, 2
• Alternatively, bronchoconstriction (≥20% reduction in FEV1) with methacholine challenge supports asthma diagnosis 2
• Patients typically have variable symptoms with wheezing and airway hyperresponsiveness 2

COPD Pattern

• Persistent obstruction: FEV1/FVC ratio <70% that does NOT reverse with bronchodilators (change <12% or <200 mL) 2
• Typically occurs in patients >40 years with significant smoking history (>10 pack-years) or biomass exposure 3
• Progressive dyspnea and chronic cough are characteristic 2

Asthma-COPD Overlap

• Demonstrates both reversibility AND persistent baseline obstruction 2, 3
• Defined as current asthma with post-bronchodilator FEV1/FVC <0.7 in patients >40 years 3, 4
• These patients have significantly worse prognosis than either disease alone, with higher exacerbation rates, faster FEV1 decline, and reduced life expectancy 3, 4, 5
• Late-onset asthma overlap (after age 40) has worse prognosis than early-onset, with FEV1 decline of 49.6 mL/year versus 27.3 mL/year 4

Step 3: Grade Severity Using FEV1 Percent Predicted

Use FEV1 percent predicted to grade severity, NOT the FEV1/FVC ratio 1

European Respiratory Society Classification

• Mild: FEV1 ≥70% predicted 1
• Moderate: FEV1 60-69% predicted 1
• Moderately severe: FEV1 50-59% predicted 1
• Severe: FEV1 35-49% predicted 1
• Very severe: FEV1 <35% predicted 1

GOLD Classification (Alternative)

• Mild: FEV1 ≥80% predicted 1
• Moderate: FEV1 50-79% predicted 1
• Severe: FEV1 30-49% predicted 1
• Very severe: FEV1 <30% predicted 1

Step 4: Assess Additional Prognostic Parameters

• Measure inspiratory capacity (IC) and diffusing capacity (DLCO) - these are important mortality predictors beyond FEV1 alone 1
• FEV1 percent predicted correlates poorly with individual symptoms and should not be used alone to predict clinical severity 6
• Consider lung hyperinflation and expiratory flow limitation for complete assessment 6

Step 5: Interpret Serial Changes Over Time

• Short-term changes: >5% change in FEV1 or FVC indicates significant variation 1
• Week-to-week changes: >12% in FEV1 or >11% in FVC suggests meaningful change 1
• Year-to-year changes: >15% in FEV1 indicates clinically meaningful progression 1
• Interpret sequential measurements relative to the individual's baseline, not just predicted values 6

Critical Pitfalls to Avoid

• Never confirm restrictive disease without measuring total lung capacity (TLC) - spirometry alone has poor positive predictive value for restriction 1
• In patients with suspected obstruction but preserved FEV1/FVC ratio, measure slow vital capacity (SVC) or inspiratory vital capacity (IVC), as FVC may underestimate true vital capacity due to small airway collapse, particularly in patients <60 years and those with BMI >30 kg/m² 6
• Do not use FEV1/FVC ratio to grade severity - only use it to identify obstruction 1
• Be aware that race- and ethnicity-specific equations may perform better for matching physiologic features but worse for predicting mortality, requiring careful consideration of individual context 7
• For borderline or gray zone results, additional testing or clinical assessment is needed, as PFT is only one tool among many 7

Special Populations

Preschool Children

• Bronchodilator responsiveness has better diagnostic accuracy than baseline measurements for asthma diagnosis 7
• Rint, forced oscillation technique (FOT at 5 Hz), and plethysmographic sRaw have similar diagnostic profiles 7
• Significant overlap exists between healthy children and those with mild asthma, requiring clinical judgment and atopy testing 7

Patients Unable to Perform Standard Spirometry

• FEV6 can substitute for FVC when full vital capacity maneuvers cannot be completed 6
• In neuromuscular disease, functional residual capacity (FRC) becomes the most reliable parameter as it requires only passive cooperation 6