What is the cutoff FEV1 (Forced Expiratory Volume in 1 second)/FVC (Forced Vital Capacity) ratio to diagnose asthma?

FEV1/FVC Ratio Cutoff for Diagnosing Asthma

The cutoff for FEV1/FVC ratio to diagnose asthma should be below the lower limit of normal (LLN) or <80% when LLN is not available. 1

Diagnostic Criteria for Airflow Obstruction in Asthma

Spirometry is a crucial component in the diagnostic workup of suspected asthma. The European Respiratory Society (ERS) clinical practice guidelines strongly recommend performing spirometry as part of the diagnostic evaluation of asthma.

Primary Cutoff Values:

• Preferred approach: FEV1/FVC below the lower limit of normal (LLN)
• Alternative when LLN unavailable: FEV1/FVC <80%

The task force strongly recommends using LLN to define abnormal spirometry but accepts a fixed cut-off of 80% where LLN is not available, as this reasonably approximates LLN in most cases 1.

Important Considerations:

• Diagnostic accuracy: Studies using a fixed cutoff of FEV1/FVC <80% showed low sensitivity (0.12-0.52) but moderate to high specificity (0.72-0.93) for asthma diagnosis 1

• False positives: Using a fixed cutoff of 80% for FEV1/FVC can falsely identify airflow obstruction in approximately 6.4% of children compared to using LLN 1

• Normal spirometry doesn't exclude asthma: Due to the variable nature of asthma, spirometry may be normal when symptoms are controlled, resulting in poor sensitivity for ruling out asthma 1

Bronchodilator Response Testing

When spirometry shows airflow obstruction (FEV1/FVC < LLN or <80%), bronchodilator response (BDR) testing should be performed:

• Positive BDR: An increase in FEV1 ≥12% and/or ≥200 mL following inhalation of a short-acting β2-agonist is diagnostic of asthma 1
• Negative BDR: Does not exclude asthma 1

Practical Application

1. First-line test: Perform spirometry with FEV1/FVC ratio assessment
2. If FEV1/FVC < LLN or <80%: Proceed with bronchodilator response testing
3. If normal spirometry but strong clinical suspicion: Consider:
   • Repeat testing during symptomatic periods
   • Additional tests like FeNO measurement
   • Trial of controller medication with objective reassessment

Common Pitfalls to Avoid

• Single measurement limitation: Due to the variable nature of asthma, a one-time normal spirometry result does not exclude asthma; serial measurements may be required 1

• Technique errors: Not all patients can perform a sufficient FVC maneuver, which may result in a falsely normal FEV1/FVC ratio 1

• Age considerations: Fixed cutoffs may be less accurate in certain age groups; LLN is preferred when available 1

• Pretest probability impact: The diagnostic accuracy of spirometry depends on the pretest probability of asthma, with different optimal cutoffs depending on whether testing is done in a high-risk or screening population 2

By using the FEV1/FVC ratio below LLN (or <80% when LLN is unavailable) as the cutoff for diagnosing asthma, clinicians can appropriately identify airflow obstruction while recognizing the limitations of spirometry as a standalone diagnostic test for asthma.