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

Diagnostic Criteria for Allergic Bronchopulmonary Aspergillosis (ABPA)

According to the 2024 revised ISHAM-ABPA working group clinical practice guidelines, ABPA diagnosis requires predisposing conditions or compatible clinical presentation, two essential components, and at least two additional components from a defined list. 1

Diagnostic Criteria Framework

Predisposing Conditions

• Asthma
• Cystic fibrosis
• Chronic obstructive lung disease
• Bronchiectasis
• Or a compatible clinico-radiological presentation

Essential Components (both required)

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

Other Components (any two required)

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

Important Clinical Considerations

• Pathognomonic Finding: High-attenuation mucus on CT confirms ABPA diagnosis even if all other criteria are not fulfilled 1, 2
• A positive type 1 skin test is acceptable when Aspergillus-IgE testing is unavailable 1, 2
• Serum total IgE <500 IU·mL⁻¹ may be acceptable if all other criteria are fulfilled 1
• Elevated IgE against recombinant Aspergillus antigens (rAsp f1, f2, and f4) supports the diagnosis and can be used as another diagnostic component 1, 2
• Compatible clinical presentations include expectoration of mucus plugs, finger-in-glove opacities, fleeting opacities on chest radiograph, and lung collapse 1

Radiological Classification of ABPA

The 2024 ISHAM guidelines classify ABPA into five radiological categories 2:

1. ABPA-S (Serological): ABPA with no bronchiectasis
2. ABPA-B: ABPA with bronchiectasis
3. ABPA-MP: ABPA with mucus plugging but without high-attenuation mucus
4. ABPA-HAM: ABPA with high-attenuation mucus
5. ABPA-CPF: ABPA with chronic pleuropulmonary fibrosis

Diagnostic Algorithm

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

Clinical Pearls and Pitfalls

• Pitfall: Relying solely on serological markers without radiological confirmation can lead to misdiagnosis
• Pitfall: Failing to recognize high-attenuation mucus as a pathognomonic finding that confirms ABPA even in the absence of other criteria
• Pearl: The HAM classification most consistently predicts immunological severity in ABPA 3
• Pearl: Central bronchiectasis and high-attenuation mucus are independent predictors of recurrent relapses 3
• Pitfall: Missing ABPA diagnosis in patients with a history of tuberculosis, as post-tuberculous destructive lesions can mask or mimic ABPA findings 4

Distinguishing ABPA from ABPM

For allergic bronchopulmonary mycosis (ABPM) caused by non-Aspergillus fungi, similar criteria apply 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