What is the initial test for detecting aspergillus antigen or antibody in asthmatic patients?

Initial Testing for Aspergillus in Asthmatic Patients

The initial test for detecting Aspergillus sensitization in asthmatic patients is Aspergillus fumigatus-specific IgE (serum blood test), which is preferred over skin testing due to its superior sensitivity (99-100% vs 88-94%) and practical advantages. 1

Screening Approach

Who Should Be Screened

• All asthmatic adults seeking tertiary care should be evaluated for A. fumigatus sensitization, as approximately 37% of sensitized individuals can develop ABPA 1
• Children should only be screened if they have difficult-to-treat asthma 1
• The rationale is that ABPA can occur even in mild asthmatic patients and carries high risk of progression to bronchiectasis if undetected 1

Primary Screening Test: Aspergillus-Specific IgE (Antibody)

A. fumigatus-specific IgE is the preferred initial screening test with the following characteristics: 1

• Cut-off value: ≥0.35 kUA/L (fluorescent enzyme immunoassay/FEIA) 1
• Sensitivity: 99-100% for detecting Aspergillus sensitization 1
• Practical advantages: Patients on antihistamines do not need to stop treatment, and quality laboratory performance data is widely available 1
• Cross-reactivity benefit: Can detect sensitization to other Aspergillus species, especially A. flavus 1

Alternative: Skin Prick Test

• Skin prick testing can be performed additionally or if fungus-specific IgE is unavailable 1
• Sensitivity is lower (88-94%) compared to IgE testing 1
• Requires weal >3 mm diameter with surrounding erythema 1
• Intradermal skin tests are more sensitive than standard skin prick but interpretation is difficult and mainly used in academic centers 1

If Aspergillus Sensitization is Detected: Further Testing

Once A. fumigatus-specific IgE is positive, additional tests are needed to diagnose ABPA: 1

Essential Additional Tests (Antibodies and Markers)

1. Total serum IgE

   • Cut-off: ≥500 IU/mL (updated from previous ≥1000 IU/mL for higher sensitivity of 98% vs 91%) 1
   • A normal IgE in corticosteroid-naïve patients makes ABPA highly unlikely 1

2. A. fumigatus-specific IgG (antibody)

   • Cut-off: ≥27-40 mgA/L depending on population 2
   • Serves as confirmatory test, not primary screening 1
   • Can be elevated in other conditions (aspergilloma, chronic pulmonary aspergillosis), requiring clinical correlation 2

3. Peripheral blood eosinophil count

   • Used as second-line test; normal counts do not exclude ABPA 1

Important Caveats and Pitfalls

Cross-Reactivity Issues

• Crude Aspergillus antigens may misclassify sensitization due to cross-reactivity with other fungi (Alternaria, Candida, Cladosporium, Malassezia, Trichophyton) 3
• Recombinant A. fumigatus antigens (rAsp f1 and f2) can help differentiate true sensitization from cross-sensitization 3
• High probability of misclassification exists if non-standardized assays are used 2

Testing Limitations

• Aspergillus-specific IgG has declined in diagnostic performance with older techniques (Ouchterlony diffusion now only 43% sensitive vs historical 70-90%) 1
• ELISA for IgG has better sensitivity (41-46%) than counter immunoelectrophoresis (15%) 1
• Sputum culture for A. fumigatus is supportive but not diagnostic (can be isolated in other diseases) 1

Timing Considerations

• Use the earliest available total IgE test, as IgE concentration declines significantly with corticosteroid treatment 1
• Re-evaluate sensitization if there is unexplained deterioration in asthma control 1