What indicates a positive test for asthma?

Positive Tests for Asthma Diagnosis

A positive test for asthma requires objective evidence of variable airflow limitation, with the gold standard being a bronchodilator reversibility test showing an increase in FEV1 of ≥12% and ≥200mL from baseline after administration of a bronchodilator. 1

Primary Diagnostic Tests for Asthma

1. Bronchodilator Reversibility Testing

• Positive result: Increase in FEV1 of ≥12% and ≥200mL from baseline after bronchodilator administration 1, 2
• Procedure:
  • Perform baseline spirometry
  • Administer short-acting bronchodilator (typically 400μg salbutamol/albuterol)
  • Repeat spirometry after 15 minutes
• High specificity (90-98%) but lower sensitivity (35-36%) for asthma diagnosis 1
• Two puffs of albuterol appear to be as effective as four puffs for determining bronchodilator responsiveness 3

2. Peak Expiratory Flow (PEF) Variability

• Positive result: Diurnal variation >10% in adults or >13% in children over 1-2 weeks 1
• Procedure:
  • Patient records PEF measurements twice daily (morning and evening) for 2 weeks
  • Calculate average daily diurnal variation: (day's highest PEF - day's lowest PEF) ÷ mean PEF for the day
• Less reliable than spirometry but useful for monitoring 1

3. Bronchial Challenge Testing

• Methacholine Challenge Test:

  • Positive result: PC20 (provocative concentration causing 20% fall in FEV1) ≤8 mg/mL 1, 4
  • High sensitivity but lower specificity than bronchodilator testing 1
  • Most useful when pretest probability of asthma is 30-70% 1

• Exercise Challenge Test:

  • Positive result: Fall in FEV1 >10% from baseline after standardized exercise 1
  • Particularly useful for diagnosing exercise-induced asthma 1

4. Fractional Exhaled Nitric Oxide (FeNO)

• Positive result: FeNO value ≥25 ppb in a child with asthma symptoms 1
• Indicates eosinophilic airway inflammation
• Supports diagnosis of asthma when combined with clinical symptoms 1, 5

Diagnostic Algorithm

1. First-line tests (should be performed in all patients with suspected asthma):

   • Spirometry with bronchodilator reversibility testing
   • FeNO measurement

2. If spirometry is abnormal (FEV1 or FEV1/FVC below lower limit of normal):

   • Perform bronchodilator reversibility testing
   • If positive (≥12% and ≥200mL improvement), asthma is confirmed
   • If negative, consider additional testing (FeNO, challenge testing)

3. If spirometry is normal but symptoms persist:

   • Measure FeNO (≥25 ppb supports asthma diagnosis)
   • Consider PEF variability testing
   • Consider bronchial challenge testing (methacholine or exercise)

4. For exercise-related symptoms:

   • Exercise challenge test showing >10% fall in FEV1 confirms exercise-induced asthma 1

Important Considerations

• No single test can definitively diagnose asthma in all cases; multiple tests may be required 1
• Variable airflow limitation is the hallmark of asthma 1
• European Respiratory Society guidelines recommend at least two objective test results be abnormal to confirm asthma diagnosis 1
• Bronchodilator reversibility has high specificity but lower sensitivity for asthma diagnosis 1, 6
• A positive methacholine challenge test may occasionally be falsely positive after respiratory infections, in very young or old patients, in smokers, or in patients with allergic rhinitis without asthma 4

Common Pitfalls

• Relying solely on symptoms without objective testing leads to misdiagnosis 1, 5
• Studies show 33.1% of patients with physician-diagnosed asthma have no evidence of asthma on pulmonary function tests 5
• A negative bronchodilator test does not rule out asthma due to its low sensitivity 1, 7
• Methacholine challenge testing has limited utility in patients with baseline airway obstruction 1
• Failure to recognize that a large bronchodilator response strongly suggests asthma, but lack of reversibility does not disprove it 8

By using this systematic approach to testing, clinicians can accurately diagnose asthma and distinguish it from other respiratory conditions that may present with similar symptoms.