Imaging Guidelines for Allergic Bronchopulmonary Aspergillosis (ABPA)

Preferred Imaging Modality

• High‑resolution computed tomography (HRCT) with 1.25–1.5 mm thin sections and intravenous contrast is the gold‑standard technique for diagnosing and monitoring ABPA, allowing optimal visualization of vessels and disease extent. 1
• Contrast‑enhanced CT (CT angiography) performed at baseline is mandatory to accurately assess bronchiectasis distribution and vascular anatomy. 1

Pathognomonic and Key Diagnostic Findings

• High‑attenuation mucus (HAM) on non‑contrast CT has 100 % specificity for ABPA; its presence confirms the diagnosis even when other criteria are incomplete. [1][2]
• HAM is detected in approximately 35–36 % of patients with ABPA and should be evaluated on mediastinal windows. 1

• When HAM is identified, ABPA is diagnosed regardless of fulfillment of additional clinical or serological criteria. 3

• Central bronchiectasis affecting the upper and middle lobes bilaterally is the hallmark radiologic feature, observed in 63–89 % of ABPA cases. [1][2]

• Mucoid impaction (mucus plugs) occurs in 59–86 % of patients and is a core element of the radiologic diagnosis. The plugs appear as branching tubular or nodular opacities filling dilated bronchi. [1][2]

• Centrilobular nodules with a tree‑in‑bud pattern are present in about 86 % of ABPA patients. [1][2]

• Consolidation is seen in 17–43 % of cases and may represent mucus‑filled bronchiectatic cavities rather than true air‑space disease. [1][2]

• Chronic pleuropulmonary fibrosis (ABPA‑CPF) is diagnosed when two or more of the following are present: pulmonary fibrosis, fibro‑cavitary lesions, fungal ball, or pleural thickening. [1][2]
• Differentiation from chronic pulmonary aspergillosis (CPA) requires immunologic profiling: markedly elevated total IgE and eosinophilia favor ABPA, whereas predominant Aspergillus‑specific IgG without marked IgE elevation favors CPA. [1][2]

Radiological Classification (2024 ISHAM‑ABPA Working Group)

• ABPA‑S (Serological) – No bronchiectasis on imaging. [1][2]
• ABPA‑B (Bronchiectasis) – Presence of bronchiectasis. [1][2]
• ABPA‑MP (Mucus Plugging) – Mucus plugs without HAM; patients with both bronchiectasis and mucus plugging are placed in this category because of greater immunologic severity. [1][2]
• ABPA‑HAM – High‑attenuation mucus identified (pathognomonic). [1][2]
• ABPA‑CPF – Two or more features of chronic pleuropulmonary fibrosis present. [1][2]

Imaging Algorithm for Diagnosis (ISHAM‑ABPA Working Group)

• Initial evaluation – Perform thin‑section HRCT (1.25–1.5 mm) with intravenous contrast in every patient with suspected ABPA based on clinical and serologic findings. 1
• Systematically assess for:
  
  • Central bronchiectasis (upper/middle‑lobe predominance), mucus plugging, and HAM on both lung and mediastinal windows. [1][2]
  • Centrilobular nodules, tree‑in‑bud pattern, consolidation, and atelectasis. [1][2]

Follow‑Up Imaging Strategy

• Routine treatment monitoring should use chest radiographs rather than CT to limit cumulative radiation exposure. [4][1]
• Repeat HRCT is reserved for:
  
  • New hemoptysis requiring evaluation of vascular complications.
  • Suspected treatment failure or marked clinical deterioration.
  • Re‑assessment at 3–6 months when clinical status changes. [4][1]
• When HRCT is necessary for follow‑up, low‑dose CT protocols may be employed to reduce radiation dose. 4

Imaging Modalities Not Recommended

• PET scanning – Not useful for ABPA diagnosis; Aspergillus infection frequently produces false‑positive FDG uptake that mimics malignancy. 4

Common Pitfalls and Recommendations

• Pitfall – Relying on chest X‑ray alone

  • Recommendation: Obtain HRCT whenever ABPA is suspected because chest X‑ray is normal in ~50 % of cases and misses critical findings.

• Pitfall – Assuming bronchiectasis is only central

  • Recommendation: Recognize that peripheral bronchiectasis occurs in ~30 % of patients; do not exclude ABPA based on peripheral distribution.

• Pitfall – Misinterpreting consolidation on X‑ray

  • Recommendation: Correlate with HRCT, which often reveals mucus‑filled bronchiectatic cavities rather than true consolidation.

• Pitfall – Missing high‑attenuation mucus

  • Recommendation: Always review mediastinal windows; HAM is 100 % specific for ABPA. [1][2]

• Pitfall – Excessive radiation from repeated CT

  • Recommendation: Use chest radiographs for routine follow‑up; reserve CT for hemoptysis, treatment failure, or significant clinical change. [4][1]

Radiological Evaluation for Allergic Bronchopulmonary Aspergillosis (ABPA)

Baseline Imaging: Thin-Section Chest CT

• The European Respiratory Society recommends performing thin-section chest CT (1.25-1.5 mm slice thickness) at baseline for all suspected ABPA cases to identify bronchiectasis, mucus plugging, and high-attenuation mucus, with a sensitivity of 35% and specificity of 100% for high-attenuation mucus 5, 6
• Thin-section CT is mandatory at baseline with contrast administration (CT angiography with intravenous contrast) to visualize vessels and assess bronchiectasis extent, according to the European Respiratory Society 7
• The European Respiratory Society suggests carefully weighing radiation exposure against diagnostic necessity in children 8
• Central bronchiectasis (bilateral, predominantly upper and middle lobes) is the hallmark finding, with cylindrical or cystic bronchiectasis involving central airways, as recommended by the European Respiratory Society 5, 9
• High-attenuation mucus is pathognomonic when present, with mucus visually denser than paraspinal muscles on non-contrast CT, and confirms ABPA diagnosis even if other criteria are incomplete, according to the European Respiratory Society 5, 10, 11

Radiological Classification System

• The 2024 ISHAM-ABPA Working Group established five CT-based categories, including ABPA-S (Serological), ABPA-B (Bronchiectasis), ABPA-MP (Mucus Plugging), ABPA-HAM, and ABPA-CPF (Chronic Pleuropulmonary Fibrosis), as recommended by the European Respiratory Society 8, 9, 12

Follow-Up Imaging Strategy

• The European Respiratory Society recommends using chest radiographs, not CT, for treatment monitoring, to minimize cumulative radiation exposure, especially with multiple follow-ups 5, 6, 7
• Repeat CT is only recommended for new hemoptysis, suspected treatment failure, or significant clinical deterioration, according to the European Respiratory Society 7

Imaging Modalities NOT Recommended

• The European Respiratory Society does not recommend magnetic resonance imaging (MRI) for routine diagnosis, despite being radiation-free, due to no significant diagnostic advantage over readily available chest CT 5, 6
• The European Respiratory Society also does not recommend PET scanning for ABPA diagnosis, as aspergillosis causes false-positive PET scans mimicking malignancy 7

Diagnostic Criteria for Allergic Bronchopulmonary Aspergillosis (ABPA) in Children

Clinical Evaluation and Diagnosis

• The European Respiratory Society recommends a systematic approach for diagnosing ABPA in children, focusing on clinical, radiological, and immunological evaluations, with mandatory demonstration of fungal sensitization and serum total IgE ≥500 IU·mL−1 plus two additional criteria 13
• Children with cystic fibrosis should be investigated for ABPA, as approximately 7% develop ABPA 14
• The American Thoracic Society suggests assessing for characteristic symptoms, including episodic wheezing and bronchial obstruction, cough and breathlessness, expectoration of brown plugs or flecks, pyrexia, fatigue, and malaise 14, 15

Laboratory Investigations

• The European Respiratory Society recommends serum total IgE ≥500 IU·mL−1 and Aspergillus fumigatus-specific IgE as mandatory criteria for ABPA diagnosis 13
• Additional tests, such as Aspergillus fumigatus-specific IgG, peripheral blood eosinophilia, and suggestive imaging findings, should be completed to support the diagnosis 13

Radiological Evaluation

• Chest CT is crucial for diagnosing ABPA in children, but radiation dosage should be carefully considered 16
• Characteristic radiological findings include central bronchiectasis, transient pulmonary opacities, high-attenuation mucus, mucus plugging, band shadows, and "ring sign" or parallel "tram lines" 14, 16, 17

Classification and Clinical Considerations

• The European Respiratory Society suggests classifying ABPA into different categories, including ABPA-S, ABPA-B, ABPA-MP, ABPA-HAM, and ABPA-CPF, based on radiological findings 16, 17
• Clinicians should be aware of the potential for atypical presentations of ABPA in children, including mass-like lesions in the lung, and consider alternative diagnoses, such as immunodeficiencies, ciliary disorders, and mycobacterial infection 15, 18

Chapter: Evidence‑Based Definition, Diagnosis, and Screening of Allergic Bronchopulmonary Aspergillosis (ABPA)

Definition & Epidemiology

• ABPA is a complex pulmonary disorder caused by an exaggerated type‑2 immune response to Aspergillus fumigatus colonizing the airways of patients with chronic lung disease, most commonly asthma or cystic fibrosis. – ISHAM ABPA Working Group 19
• ABPA represents an asthma endotype and a treatable trait in both cystic fibrosis and non‑CF bronchiectasis. – ISHAM ABPA Working Group 20
• ABPA may also develop in patients with bronchiectasis, chronic obstructive pulmonary disease, or even without any recognized predisposing condition. – ISHAM ABPA Working Group 20

Etiology

• Aspergillus fumigatus is the primary causative organism in classic ABPA; other fungi can cause related allergic bronchopulmonary mycoses (ABPM). – ISHAM ABPA Working Group 19

Clinical Manifestations

• Chronic central bronchiectasis (bilateral, upper/middle‑lobe predominant) is a characteristic radiologic feature of ABPA. – ISHAM ABPA Working Group 19

Diagnostic Criteria (2024 ISHAM‑ABPA Working Group)

• Diagnosis requires (1) a compatible predisposing condition, (2) documented A. fumigatus sensitization, (3) serum total IgE ≥ 500 IU/mL, and (4) at least two of the following: A. fumigatus‑specific IgG, peripheral eosinophilia, or suggestive imaging. – ISHAM ABPA Working Group 19

Laboratory Thresholds

• The serum total IgE cutoff was lowered from 1,000 IU/mL to ≥ 500 IU/mL to improve sensitivity. – ISHAM ABPA Working Group 21
• Elevated A. fumigatus‑specific IgE is mandatory and is superior to skin testing for confirming sensitization. – ISHAM ABPA Working Group 21
• Elevated A. fumigatus‑specific IgG measured by enzyme immunoassay provides additional diagnostic support and is superior to immunoprecipitation methods. – ISHAM ABPA Working Group 21

Supporting Laboratory Findings

• Peripheral blood eosinophilia is an accepted additional diagnostic criterion. – ISHAM ABPA Working Group 19
• Imaging findings compatible with ABPA (e.g., central bronchiectasis, high‑attenuation mucus) serve as an additional criterion. – ISHAM ABPA Working Group 19

Recommended Laboratory Tests

Screening Recommendations

• Systematic screening for A. fumigatus sensitization is advised in all newly diagnosed adult asthmatics evaluated at tertiary centers. – ISHAM ABPA Working Group 19
• Screening for ABPA should also be performed in children with difficult‑to‑treat asthma. – ISHAM ABPA Working Group 19

Diagnostic Challenges in Cystic Fibrosis

• The same ISHAM diagnostic criteria (total IgE ≥ 500 IU/mL, specific IgE, plus two additional criteria) apply to cystic fibrosis patients, although baseline bronchiectasis and mucus plugging can complicate interpretation. – ISHAM ABPA Working Group 19
• The presence of extensive baseline bronchiectasis in CF makes the identification of central bronchiectasis a key discriminating feature. – ISHAM ABPA Working Group 20

Clinical Importance of Early Diagnosis

• Early identification and treatment of ABPA are essential to prevent irreversible bronchiectasis, pulmonary fibrosis, and long‑term loss of lung function. – ISHAM ABPA Working Group 19

Guideline Updates (2024 ISHAM)

• The 2024 ISHAM guidelines formally lowered the total IgE diagnostic threshold to ≥ 500 IU/mL, enhancing sensitivity without compromising specificity. – ISHAM ABPA Working Group 21

Strength of evidence: The guideline statements above are based on expert consensus and systematic review of available data; specific grades of recommendation were not detailed in the source article.