What are the diagnostic criteria for allergic bronchopulmonary aspergillosis (ABPA) in a patient with asthma or cystic fibrosis?

Diagnostic Criteria for ABPA

The 2024 revised ISHAM-ABPA working group criteria require a predisposing condition (or compatible presentation), BOTH essential components (A. fumigatus-specific IgE ≥0.35 kUA·L⁻¹ AND serum total IgE ≥500 IU·mL⁻¹), plus ANY TWO additional components from: positive A. fumigatus-specific IgG, blood eosinophils ≥500 cells·μL⁻¹, or characteristic imaging findings. 1

Predisposing Conditions or Clinical Presentation

The diagnosis should be suspected in patients with: 1, 2

• Asthma (most common)
• Cystic fibrosis
• COPD
• Bronchiectasis
• Compatible clinico-radiological presentation even without the above conditions, including:
  • Expectoration of brown mucus plugs 2
  • Poorly controlled asthma despite therapy 2
  • Recurrent pulmonary infiltrates or fleeting opacities 1, 2
  • Finger-in-glove opacities 1
  • Lung collapse 1

Essential Components (BOTH Required)

1. A. fumigatus-Specific IgE

• Threshold: ≥0.35 kUA·L⁻¹ 1, 2
• A positive type 1 skin test to Aspergillus is acceptable when IgE testing is unavailable 1, 2

2. Serum Total IgE

• Threshold: ≥500 IU·mL⁻¹ 1
• This lower threshold (500 vs. 1000 IU·mL⁻¹) is more sensitive 1
• Important caveat: Total IgE <500 IU·mL⁻¹ may be acceptable if all other criteria are fulfilled, particularly in: 1, 2
  • Patients with prior glucocorticoid treatment
  • Elderly patients
  • Those with constitutively low baseline IgE

Additional Components (ANY TWO Required)

1. Positive A. fumigatus-Specific IgG

• Use population-specific cut-offs when available: 1, 2
  • India: ≥27 mgA·L⁻¹
  • Japan: ≥60 mgA·L⁻¹
  • UK: ≥40 mgA·L⁻¹
• Use manufacturer recommendations when population data unavailable 1
• Can be detected by lateral flow assays or enzyme immunoassays 1

2. Blood Eosinophil Count

• Threshold: ≥500 cells·μL⁻¹ 1
• Historical values are acceptable (doesn't need to be current) 1

3. Characteristic Imaging Findings

Thin-section chest CT (recommended at baseline with 92.3% consensus) showing: 1, 2

• Central bronchiectasis
• Mucus plugging
• High-attenuation mucus (HAM) - this is pathognomonic and confirms ABPA diagnosis even if other criteria are incomplete 1, 3, 2

OR

Chest radiograph showing: 1

• Fleeting opacities consistent with ABPA (high specificity)
• Finger-in-glove opacities

Diagnostic Algorithm

1. Screen high-risk patients: All asthmatics with Aspergillus sensitization and approximately 7% of CF patients warrant screening 2

2. Confirm BOTH essential components:

   • A. fumigatus-specific IgE ≥0.35 kUA·L⁻¹
   • Total IgE ≥500 IU·mL⁻¹

3. Identify ANY TWO additional components:

   • A. fumigatus-specific IgG (using appropriate cut-offs)
   • Eosinophils ≥500 cells·μL⁻¹
   • Characteristic CT or radiograph findings

4. Special consideration: If high-attenuation mucus is present on CT, this alone confirms ABPA even without all other criteria 1, 3

Additional Supportive Tests

• Elevated IgE against recombinant Asp f1, f2, and f4 supports diagnosis and can be used as another diagnostic component 1, 3
• Sputum fungal culture is suggested during evaluation to identify species or guide therapy (but not required for diagnosis) 1
• Serum galactomannan is NOT recommended for diagnosing ABPA 1
• Bronchoscopy is NOT routinely recommended unless diagnosis is uncertain, ABPM is suspected, unexplained hemoptysis occurs, or chronic infection is suspected 1, 2

Critical Pitfalls to Avoid

• Don't dismiss ABPA with total IgE <500 IU·mL⁻¹ if the patient has been on corticosteroids or is elderly 1, 2
• Don't overlook high-attenuation mucus on CT - this single finding is pathognomonic and diagnostic 1, 3
• Don't use skin testing preferentially - serum A. fumigatus-specific IgE is superior 1
• Don't rely on immunoprecipitation for IgG - enzyme immunoassay is superior 1
• In children, carefully consider radiation exposure when ordering thin-section chest CT 2

Distinguishing ABPA from ABPM

If the clinical picture suggests ABPA but A. fumigatus-IgE is <0.35 kUA·L⁻¹, consider ABPM (caused by fungi other than A. fumigatus): 1, 3

• Requires elevated fungus-specific IgE (to the causative fungus)
• Requires total IgE ≥500 IU·mL⁻¹
• Requires ANY TWO of: fungus-specific IgG, eosinophils ≥500 cells·μL⁻¹, two sputum cultures (or one BAL) growing the causative fungus, or characteristic imaging
• Absence of elevated IgE against rAsp f1, f2, and f4 strongly supports ABPM over ABPA 1