What is the clinical classification of Allergic Bronchopulmonary Aspergillosis (ABPA)?

ABPA Clinical Classification

The 2024 revised ISHAM-ABPA working group guidelines establish a dual classification system for ABPA: a clinical classification with five categories (acute ABPA, response, remission, treatment-dependent ABPA, and advanced ABPA) and a radiological classification with five categories (ABPA-S, ABPA-B, ABPA-MP, ABPA-HAM, and ABPA-CPF). 1

Clinical Classification System

The clinical classification framework replaces the previous numbered staging system (0-6) because patients do not necessarily progress sequentially through stages, and the old system counterintuitively placed remission (stage 4) after exacerbation (stage 3). 1

Five Clinical Categories:

1. Acute ABPA 1

• Newly diagnosed: Previously undiagnosed ABPA/M meeting diagnostic criteria
• Exacerbation: In diagnosed ABPA patients with:
  • Sustained clinical worsening (>14 days) OR radiological worsening, AND
  • Increase in serum total IgE by ≥50% from baseline during clinical stability, AND
  • Exclusion of other causes (asthma exacerbation or infective bronchiectasis exacerbation)

Important distinction: Asthma exacerbation presents with worsening respiratory symptoms for ≥48 hours WITHOUT immunological or radiological deterioration of ABPA. Infective/bronchiectasis exacerbation shows clinical deterioration for ≥48 hours with increased cough, breathlessness, sputum changes, purulence, fatigue, malaise, fever, or hemoptysis WITHOUT immunological or radiological deterioration of ABPA. 1

2. Response 1

• Symptomatic improvement by ≥50% (on Likert or visual analogue scale) after 8 weeks, AND
• Major radiological improvement (>50% reduction in opacities) OR decline in serum total IgE by ≥20% after 8 weeks of treatment

3. Remission 1

• Sustained (≥6 months) clinico-radiological improvement, off glucocorticoids, AND
• Lack of rise in serum total IgE by ≥50% from baseline during clinical stability
• Patients on biological agents or long-term antifungal agents may also be considered in remission if meeting these criteria 1

4. Treatment-Dependent ABPA 1

• Two or more consecutive ABPA/M exacerbations, each within 3 months of stopping glucocorticoids, OR
• Worsening respiratory symptoms AND worse imaging or rise in serum total IgE by 50% within 4 weeks of tapering oral steroids on two separate occasions

5. Advanced ABPA 1

• Extensive bronchiectasis (≥10 segments) due to ABPA/M on chest imaging, AND
• Cor pulmonale or chronic type 2 respiratory failure

Radiological Classification System

The radiological classification is prognostically important, as the extent of bronchiectasis, high-attenuation mucus (HAM), and fungal balls independently predict recurrent ABPA exacerbations. 1 Central bronchiectasis (usually bilateral) is the predominant pattern, though both central and peripheral bronchiectasis occur in up to 40% of lobes. 1

Five Radiological Categories:

1. ABPA-S (Serological ABPA) 1, 2, 3

• ABPA with no bronchiectasis on imaging

2. ABPA-B (ABPA with Bronchiectasis) 1, 2, 3

• ABPA with radiological evidence of bronchiectasis

3. ABPA-MP (ABPA with Mucus Plugging) 1, 2, 3

• ABPA with mucus plugging but WITHOUT high-attenuation mucus
• Critical point: Patients with BOTH bronchiectasis and mucus plugging are classified as ABPA-MP (not ABPA-B) due to greater immunological severity associated with mucus plugging 1

4. ABPA-HAM (ABPA with High-Attenuation Mucus) 1, 2, 3

• ABPA with high-attenuation mucus on CT
• HAM is pathognomonic for ABPA and confirms diagnosis even if other criteria are not fully met 3

5. ABPA-CPF (ABPA with Chronic Pleuropulmonary Fibrosis) 1, 2

• ABPA with two or more of: pulmonary fibrosis, fibro-cavitary lesions, fungal ball, and pleural thickening
• Critical caveat: Chronic pulmonary aspergillosis complicating ABPA must be excluded in these patients 1

Additional Radiological Findings

Other common CT findings that can occur in isolation or with ABPA-B, ABPA-MP, and ABPA-HAM include: 1

• Centrilobular nodules (tree-in-bud appearance)
• Atelectasis
• Mosaic attenuation
• Consolidation

Clinical Implications

The asymptomatic stage and glucocorticoid-dependent asthma categories were removed from the new classification because they had no clear treatment implications. 1 This streamlined approach focuses on clinically actionable categories that directly guide therapeutic decisions and prognostication, prioritizing outcomes related to morbidity (bronchiectasis progression, exacerbation frequency), mortality (respiratory failure, cor pulmonale), and quality of life (symptom control, steroid dependence). 1

The dual classification system allows clinicians to simultaneously assess disease activity (clinical classification) and structural lung damage (radiological classification), providing a comprehensive framework for individualized management strategies. 1