What is the best lab test to evaluate airway inflammation?

Best Laboratory Tests for Evaluating Airway Inflammation

Induced sputum analysis is the best laboratory test for evaluating airway inflammation, providing direct measurement of inflammatory cells and their mediators in the airways. 1

Comparison of Available Tests

Induced Sputum Analysis

• Gold standard for assessing airway inflammation
• Directly measures inflammatory cells in the airways
• Differentiates between eosinophilic and neutrophilic inflammation
• Provides both differential and total cell counts
• Success rates: 80-90% in adults, lower in children (not possible in children <8 years) 1
• Requires specialized laboratory facilities and trained personnel

Fractional Exhaled Nitric Oxide (FeNO)

• Non-invasive surrogate marker for eosinophilic inflammation
• Easier to perform than sputum induction
• More suitable for primary care settings 1
• Limitations:
  • Suboptimal positive and negative predictive values for eosinophilia
  • Does not provide information about neutrophilic inflammation
  • Less useful in severe asthma where neutrophilic inflammation may predominate 1

Blood Eosinophil Count

• Simplest and most accessible test
• Indirect measure of airway inflammation
• Less accurate than sputum eosinophils 2
• May serve as a surrogate biomarker when sputum analysis is unavailable 3

Induced Sputum Analysis Procedure

1. Premedication with short-acting bronchodilator to prevent bronchospasm
2. Induction with nebulized hypertonic saline (3%, 4%, 5%) for 5 minutes each via ultrasonic nebulizer
3. Processing of expectorated sputum:
   • Dispersion using mucolytic agent (dithiothreitol)
   • Filtration through 48-μm mesh gauze
   • Centrifugation to produce cytospin
4. Analysis by counting 400 nonsquamous cells to obtain differential cell count 1

Clinical Applications

Diagnostic Value

• Identifies eosinophilic inflammation in asthma and non-asthmatic eosinophilic bronchitis (NAEB)
• Distinguishes between eosinophilic and neutrophilic inflammation
• Normal range for sputum eosinophils: upper limit of 1.9% 1
• Helps diagnose NAEB when patients have:
  • Normal chest radiograph
  • Normal spirometry
  • No evidence of variable airflow obstruction or hyperresponsiveness 1

Treatment Guidance

• Predicts corticosteroid responsiveness:
  • Eosinophilic inflammation responds well to corticosteroids
  • Neutrophilic inflammation typically does not respond to corticosteroids 4
• Guides treatment decisions in moderate to severe asthma 1
• Cost-effective in specialist settings 1

Practical Considerations

Limitations of Induced Sputum

• Requires specialized laboratory facilities
• Same-day processing needed for cell viability
• Not widely available outside academic centers
• Relative contraindication in patients with very low lung function (FEV₁ <1.0 L) 1

When to Use Alternative Tests

• Use FeNO when:

  • Sputum induction facilities unavailable
  • Patient unable to produce adequate sputum sample
  • In primary care settings
  • In children 5-8 years old (too young for sputum induction) 1

• Use blood eosinophil count when:

  • Both sputum and FeNO testing unavailable
  • As a screening tool before more specialized testing

Conclusion

For the most accurate assessment of airway inflammation, induced sputum analysis provides the most comprehensive and direct information about the type and degree of inflammation. While FeNO and blood eosinophil counts offer more accessible alternatives, they provide less complete information about the inflammatory process in the airways.