What is the best predictor of a correct asthma diagnosis based on pulmonary function tests, specifically Forced Expiratory Volume in 1 second (FEV1) and Forced Vital Capacity (FVC) measurements?

Best Predictor of Asthma Diagnosis on Pulmonary Function Testing

The best predictor of a correct asthma diagnosis in this patient is option C: FEV1 increase by more than 12% with bronchodilator. This demonstrates reversible airflow obstruction, which is the hallmark diagnostic feature of asthma and provides objective confirmation of the diagnosis 1.

Why Bronchodilator Reversibility is the Gold Standard

Bronchodilator reversibility testing directly demonstrates the defining pathophysiological characteristic of asthma—variable and reversible airflow limitation. The widely accepted diagnostic criterion is an increase in FEV1 of ≥12% AND ≥200 mL from baseline after administration of a short-acting bronchodilator (typically 400 μg salbutamol) 1.

Key Supporting Evidence:

• European guidelines explicitly state that an FEV1 increase ≥12% and/or ≥200 mL following bronchodilator administration is diagnostic of asthma 1
• This criterion has high specificity (0.90-0.98) for asthma diagnosis, meaning a positive test strongly confirms the diagnosis 1
• All major international asthma guidelines (GINA, BTS/SIGN, ATS/ERS) incorporate bronchodilator reversibility as part of the diagnostic definition of asthma 1

Why the Other Options Are Inferior

Option A: FEV1 Prediction

• FEV1 as a percentage of predicted alone does not demonstrate reversibility, which is essential for asthma diagnosis 1
• Many patients with mild or well-controlled asthma have normal baseline FEV1 values (>80% predicted), making this insensitive 1

Option B: FEV1/FVC <80% of Predicted

• This indicates airflow obstruction but does not differentiate asthma from COPD or other obstructive diseases 1
• Low sensitivity (0.12-0.52) for asthma diagnosis in children, with only moderate specificity 1
• The ratio can be normal in patients with mild asthma or when disease is well-controlled 1

Option D: FEV1/FVC Ratio Increase by More Than 10%

• While the ratio may improve with bronchodilator, this is not a standard diagnostic criterion in any major guideline 1
• The absolute change in FEV1 (option C) is the validated and recommended parameter 1

Clinical Application and Important Caveats

When to Perform Bronchodilator Testing:

• Recommended for all patients with FEV1 <80% predicted or FEV1/FVC <80% 1
• Consider testing even with normal baseline spirometry if clinical history strongly suggests asthma 1

Critical Limitations to Recognize:

• Bronchodilator reversibility <12% does NOT exclude asthma 1—sensitivity is only 0.35-0.36 in some studies 1
• The test represents a single snapshot in time; asthma's variable nature means serial measurements may be needed 1
• Avoid testing within 4 hours of short-acting bronchodilator use or 15 hours of long-acting bronchodilator use to prevent false negatives 1

Additional Diagnostic Considerations:

• In patients with normal FEV1 and negative bronchodilator response, smaller airway parameters (FEF25-75%) showing ≥35% improvement may support the diagnosis 2
• Peak flow variability ≥20% over time provides complementary evidence of variable airflow limitation 1
• Recent research suggests that even smaller improvements in FEV1 (>3.5-5.9%) combined with elevated FeNO may predict treatment response in patients with normal baseline spirometry 3, 4

The 12% and 200 mL improvement in FEV1 after bronchodilator remains the most evidence-based, guideline-supported diagnostic criterion for confirming asthma in this clinical scenario 1.