Diagnostic Approach to Asthma

Introduction to Diagnostic Methods

The National Asthma Education and Prevention Program (NAEPP) recommends spirometry for initial assessment, evaluation of treatment response, and assessment at least every 1-2 years, as it is critical for diagnosing and managing asthma 2
Spirometry demonstrates two key diagnostic features: (1) airway obstruction (reduced FEV1 and FEV1/FVC ratio), and (2) reversibility (≥12% and 200 mL increase in FEV1 after bronchodilator), which is characteristic of asthma 1, 2

Obtain history of episodic symptoms: difficulty breathing, chest tightness, cough (worse at night), wheezing, and identify triggers: exercise, viral infections, allergens, cold air, chemicals, environmental tobacco smoke 1, 2
Measure FEV1 and FEV1/FVC ratio to demonstrate obstruction, and administer bronchodilator (salbutamol 400 μg by MDI + spacer or 2.5 mg by nebulizer) to look for ≥15% change in FEV1 and at least 200 mL improvement, or ≥20% change in PEF with at least 60 L/min improvement 1, 2
If spirometry is normal but clinical suspicion remains high, serial PEF measurements over 1-2 weeks can demonstrate variability, and calculate amplitude % best: (highest - lowest)/highest × 100, with a variability ≥20% with minimum change of 60 L/min for 3 days in a week over 2 weeks being highly suggestive of asthma 1, 2, 4

A negative methacholine challenge (PC20 >8-16 mg/ml) has excellent negative predictive power (>90%) and effectively rules out asthma, but positive tests have poor specificity, with false-positive rates around 30% in patients with allergic rhinitis 3, 5
Patients may have normal spirometry between episodes of bronchospasm—serial measurements may be necessary, and ensure patients are not on intensive anti-inflammatory medications that could suppress airway responsiveness during testing 1, 5
In children aged 4-8 years, approximately 50% cannot perform PEF maneuvers adequately, limiting this diagnostic approach 4