What are the diagnostic criteria for Allergic Bronchopulmonary Aspergillosis (ABPA)?

Diagnostic Criteria for Allergic Bronchopulmonary Aspergillosis (ABPA)

According to the 2024 International Society for Human and Animal Mycology (ISHAM)-ABPA working group consensus criteria, ABPA diagnosis requires predisposing conditions or compatible clinical presentation, two essential components, and at least two additional components from a defined list. 1

Essential Diagnostic Components

ABPA diagnosis requires all of the following:

1. Predisposing conditions or compatible clinical presentation

   • Predisposing conditions: asthma, cystic fibrosis, chronic obstructive lung disease, bronchiectasis
   • Compatible presentations: expectoration of mucus plugs, finger-in-glove opacities, fleeting opacities on chest radiograph, lung collapse

2. Essential components (both required):

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

3. Other components (at least two required):

   • Positive IgG against A. fumigatus
   • Blood eosinophil count ≥500 cells·μL⁻¹ (could be historical)
   • Thin-section chest CT consistent with ABPA (bronchiectasis, mucus plugging and high-attenuation mucus) or fleeting opacities on chest radiograph consistent with ABPA

Important Considerations

• High-attenuation mucus on CT is pathognomonic and confirms ABPA diagnosis even if all other criteria are not fulfilled 1
• A positive type 1 skin test is acceptable when Aspergillus-IgE testing is unavailable
• Serum total IgE <500 IU·mL⁻¹ may be acceptable if all other criteria are fulfilled
• Elevated IgE against recombinant Aspergillus antigens (rAsp f1, f2, and f4) supports the diagnosis of ABPA and could be used as another component for diagnosis 2

Diagnostic Algorithm

1. Test for A. fumigatus-specific IgE
2. If ≥0.35 kUA·L⁻¹, measure serum total IgE
3. If total IgE ≥500 IU·mL⁻¹, proceed with:
   • A. fumigatus-specific IgG testing
   • Peripheral blood eosinophil count
   • Thin-section chest CT
   • Lung function tests

Radiological Classification

The 2024 ISHAM guidelines classify ABPA into five radiological categories 1:

1. ABPA-S (Serological): ABPA without bronchiectasis
2. ABPA-B: ABPA with bronchiectasis
3. ABPA-MP: ABPA with mucus plugging (non-hyperattenuating)
4. ABPA-HAM: ABPA with high-attenuation mucus
5. ABPA-CPF: ABPA with chronic pleuropulmonary fibrosis

Clinical Classification

ABPA is also classified clinically as 1:

• Acute ABPA: Newly diagnosed or exacerbation
• Remission: Sustained (≥6 months) clinico-radiological improvement, off glucocorticoids
• Treatment-dependent ABPA: Two or more consecutive exacerbations within 3 months of stopping glucocorticoids
• Advanced ABPA: Extensive bronchiectasis (≥10 segments) with cor pulmonale or chronic type 2 respiratory failure

Allergic Bronchopulmonary Mycosis (ABPM)

Similar criteria apply for ABPM caused by non-Aspergillus fungi, with key differences 1:

• A. fumigatus-specific IgE <0.35 kUA·L⁻¹
• Elevated fungus-specific IgE for the causative organism
• Two sputum cultures or one bronchoalveolar lavage fluid culture growing the causative fungus
• Absence of elevated IgE against rAsp f1, f2, and f4 supports ABPM over ABPA

Common Pitfalls and Caveats

• ABPA may be overlooked in patients with atypical manifestations who don't meet all standard diagnostic criteria 3
• Low serum total IgE can occur in patients with prior glucocorticoid treatment, the elderly, or those with constitutively low IgE 1
• Bronchoscopy with biopsy and washing may be helpful in detecting patients with atypical presentations 3
• In cystic fibrosis patients, diagnosis is particularly challenging due to overlapping symptoms with the underlying lung disease 4
• Combining rAspf4 IgE detection with anti-Aspergillus IgG ELISA and precipitin detection can improve diagnostic sensitivity 5