How do you differentiate asthma and Chronic Obstructive Pulmonary Disease (COPD) in spirometry?

Differentiating Asthma and COPD in Spirometry

The key spirometric difference between asthma and COPD is that COPD shows persistent airflow limitation with a post-bronchodilator FEV1/FVC ratio <0.70, while asthma typically demonstrates significant bronchodilator reversibility that may normalize the ratio. 1, 2

Key Diagnostic Criteria

COPD Spirometric Features:

• Post-bronchodilator FEV1/FVC ratio <0.70 (fixed threshold) 1, 2
• Limited bronchodilator response (though some COPD patients may show significant response) 1
• Persistent airflow limitation that doesn't normalize after bronchodilator 3
• Progressive decline in lung function over time

Asthma Spirometric Features:

• Variable airflow limitation
• Significant bronchodilator reversibility (increase in FEV1 ≥12% and ≥200mL from baseline) 3
• May have normal spirometry between exacerbations
• FEV1/FVC ratio may normalize after bronchodilator administration 4

Diagnostic Algorithm

1. Perform post-bronchodilator spirometry:

   • Essential for accurate diagnosis 3
   • Omitting bronchodilator testing leads to COPD overdiagnosis by approximately 39% 3

2. Assess FEV1/FVC ratio:

   • Post-bronchodilator FEV1/FVC <0.70 suggests COPD 1, 2
   • Normal post-bronchodilator ratio suggests asthma or other conditions

3. Evaluate bronchodilator response:

   • Calculate absolute change in FEV1 (mL)
   • Calculate percent change from baseline FEV1
   • Calculate percent change from predicted FEV1

4. Interpret bronchodilator response:

   • Increase in FEV1 ≥12% and ≥200mL from baseline suggests asthma 3, 4
   • Increase in FEV1 <9% of predicted value is more specific for COPD 4

5. Consider additional factors:

   • Pattern of obstruction (fixed vs. variable)
   • Presence of gas trapping (increased residual volume)
   • FEV1/SVC ratio if FEV1/FVC is borderline or normal but clinical suspicion is high 1

Important Considerations

• Lower limit of normal (LLN) vs. fixed ratio: While the fixed FEV1/FVC ratio of 0.70 is traditionally used, using the LLN may be more accurate, especially in older adults 2

• Borderline results: If post-bronchodilator FEV1/FVC is between 0.60-0.80, repeat spirometry in 3-6 months 2

• Asthma-COPD overlap: Some patients show features of both conditions, with persistent airflow limitation but significant bronchodilator response 3

• Diagnostic accuracy: Current spirometric criteria have limitations - the post-bronchodilator FEV1 <80% predicted with FEV1/FVC <70% has 100% sensitivity but only 38% specificity for COPD 4

• Volume responders: Patients with normal pre-bronchodilator ratio but abnormal post-bronchodilator ratio require careful monitoring 2

• Flow responders: Patients with abnormal pre-bronchodilator ratio but normal post-bronchodilator ratio may have asthma 2

Pitfalls to Avoid

1. Relying solely on pre-bronchodilator values: This leads to COPD overdiagnosis and misclassification of asthma 3

2. Neglecting clinical history: Spirometry should be interpreted alongside exposure history (smoking), symptom pattern, and age of onset 2, 5

3. Infrequent testing: Asthma may have normal spirometry between exacerbations, requiring repeated measurements 6

4. Overlooking alternative measures: In cases with normal FEV1/FVC but high clinical suspicion, consider measuring FEV1/SVC ratio 1

5. Misinterpreting bronchodilator response: Some COPD patients show significant bronchodilator response, and some asthmatics may not show reversibility during testing 1, 4

By systematically applying these criteria and considering both the pattern of airflow limitation and bronchodilator response, clinicians can more accurately differentiate between asthma and COPD using spirometry.