How do you interpret spirometry results in patients with varying demographics and medical histories, including those with respiratory diseases such as Chronic Obstructive Pulmonary Disease (COPD) or asthma?

How to Read Spirometry

Start by examining the FEV1/FVC ratio to identify obstruction, then assess FVC for restriction, and finally evaluate post-bronchodilator response to distinguish between asthma and COPD. 1, 2

Step 1: Assess the FEV1/FVC Ratio (Primary Parameter)

• FEV1/FVC < 0.7 indicates obstructive lung disease (COPD, asthma, or other obstructive conditions) 1, 2
• The FEV1/FVC ratio is the single most important parameter for identifying airflow obstruction and predicts morbidity and mortality even when FEV1 is within normal range 1
• Alternative threshold: FEV1/FVC below the 5th percentile of predicted (lower limit of normal) can be used instead of the fixed 0.7 cutoff 1
• If FEV1/FVC ≥ 0.7, obstruction is ruled out and you proceed to assess for restriction 2, 3

Step 2: Evaluate FVC for Restrictive Pattern

• If FEV1/FVC is normal but FVC < 80% predicted (or below 5th percentile), suspect restrictive disease 2, 3
• A restrictive pattern requires confirmation with total lung capacity (TLC) measurement via plethysmography, as spirometry alone cannot definitively diagnose restriction 1, 2
• If both FEV1/FVC and FVC are low, this indicates a mixed obstructive-restrictive defect 3

Step 3: Perform Post-Bronchodilator Testing (Critical for Diagnosis)

• Pre-bronchodilator spirometry can rule out COPD if FEV1/FVC ≥ 0.7, but post-bronchodilator testing is mandatory to confirm COPD diagnosis 1, 4
• Administer 400 mcg salbutamol or 80 mcg ipratropium bromide and repeat spirometry 15-20 minutes later 1, 4
• Positive bronchodilator response (≥12% AND ≥200 mL increase in FEV1 or FVC in adults) suggests asthma or partially reversible COPD 2, 4, 3
• Minimal or no response indicates fixed airway obstruction consistent with COPD 2, 5

Important Caveat on Bronchodilator Response:

• Many COPD patients show some bronchodilator response, so reversibility alone does not exclude COPD 4
• The GOLD criteria (post-bronchodilator FEV1/FVC < 0.7) are highly sensitive (100%) but not specific (38%) for distinguishing COPD from asthma 5

Step 4: Classify Severity Using FEV1 % Predicted

Once obstruction is confirmed post-bronchodilator, classify severity based on FEV1 % predicted: 1, 4

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

Step 5: Examine the Flow-Volume Loop

• Obstructive pattern: Concave ("scooped out") expiratory flow curve with reduced peak expiratory flow 2, 6
• Restrictive pattern: Convex or normal-shaped curve but with reduced volumes 2
• The flow-volume loop provides visual confirmation of the pattern identified by numerical values 6

Special Populations and Pitfalls

Discordant Pre- vs. Post-Bronchodilator Results:

• "Flow responders" (pre-BD obstruction that resolves post-BD) have increased risk of developing persistent COPD and require monitoring 1, 2
• "Volume responders" (normal pre-BD but obstructed post-BD) may indicate gas trapping and warrant further evaluation 1, 2

Common Interpretation Errors to Avoid:

• Do not rely on FVC alone when FEV1/FVC is low - FVC is often reduced more than slow vital capacity in obstruction, potentially masking the severity 1
• Use the largest available vital capacity (whether inspiratory, slow expiratory, or forced) rather than defaulting to FVC 1
• Avoid examining multiple flow parameters simultaneously (e.g., FEF25-75%) as this increases false-positive rates; stick to FEV1, FVC, and their ratio for primary interpretation 1
• Repeat spirometry within 3-6 months if post-bronchodilator FEV1/FVC falls between 0.60-0.80 to account for biological variation 4

Additional Parameters (Use Selectively):

• Peak expiratory flow (PEF) and maximum inspiratory flows can help diagnose extrathoracic airway obstruction but should not be used for primary interpretation 1
• Inspiratory capacity (IC) provides an indirect measure of hyperinflation in COPD patients and correlates with dyspnea better than FEV1 1

Integration with Clinical Context

• Always interpret spirometry alongside clinical symptoms, smoking history, and occupational exposures 4, 6
• Use the modified Medical Research Council (mMRC) dyspnea scale (0-4) to assess functional impairment beyond spirometry 1, 4
• Screen for comorbidities (cardiovascular disease, lung cancer, osteoporosis) as these significantly impact prognosis in COPD 1, 4