What are the latest guidelines for classifying Idiopathic Interstitial Lung Diseases (ILDs)?

Latest Guidelines on Idiopathic Interstitial Lung Disease (ILD) Classification

The most current classification of Idiopathic Interstitial Lung Diseases (ILDs) follows a multidisciplinary approach that integrates clinical, radiological, and pathological findings, with the 2022 Chest guidelines emphasizing the integration of IPF and fibrotic hypersensitivity pneumonitis diagnostic algorithms within a comprehensive ILD framework. 1

Current ILD Classification Framework

Major Idiopathic Interstitial Pneumonias

• Idiopathic Pulmonary Fibrosis (IPF) - Most common IIP (55% of cases) 1
• Idiopathic Nonspecific Interstitial Pneumonia (NSIP) - Second most common (25%) 1
• Respiratory Bronchiolitis-Interstitial Lung Disease (RB-ILD) - 10-15% of cases 1
• Desquamative Interstitial Pneumonia (DIP) 1
• Cryptogenic Organizing Pneumonia (COP) - 5% of cases 1
• Acute Interstitial Pneumonia (AIP) - <2% of cases 1

Rare Idiopathic Interstitial Pneumonias

• Idiopathic Lymphoid Interstitial Pneumonia (LIP) - <1% of cases 1
• Idiopathic Pleuroparenchymal Fibroelastosis - Added in the 2013 ATS/ERS update 1

Unclassifiable Idiopathic Interstitial Pneumonias

Approximately 10% of all patients with fibrotic ILD remain unclassifiable even after thorough evaluation 1, 2. Causes include:

• Inadequate clinical, radiologic, or pathologic data
• Major discordance between clinical, radiologic, and pathologic findings
• Previous therapy altering radiologic or histologic findings
• New entities or unusual variants not adequately characterized by current classification 1

Diagnostic Approach

Confidence-Based Diagnostic Framework

The American Thoracic Society recommends a confidence-based approach with categories 2:

• Confident diagnosis (≥90% certainty)
• Provisional high confidence (70-89% certainty)
• Provisional low confidence (51-69% certainty)
• Unlikely (≤50% certainty)

Multidisciplinary Discussion (MDD)

MDD is central to ILD diagnosis, requiring close communication between clinician, radiologist, and pathologist 1, 3. The 2022 guidelines emphasize that:

• MDD is a long-term and iterative process
• Even when pathologic analysis is sought, approximately 10% of fibrotic ILD cases remain unclassifiable 1
• MDD should integrate clinical, radiologic, and pathologic domains 1

Diagnostic Algorithm Components

1. Rule out known causes 2:

   • Detailed medication history and environmental exposures
   • Serological testing to exclude connective tissue diseases
   • Rule out "ILD masqueraders" (cardiac disease, pulmonary vascular disease, infection, malignancy)

2. HRCT pattern evaluation 1, 2:

   • Determine if pattern is typical for specific ILD
   • Assess for fibrosis
   • Patterns include UIP (usual interstitial pneumonia), NSIP, organizing pneumonia, etc.

3. Histopathological assessment (when needed) 1:

   • Surgical lung biopsy not required for patients with HRCT pattern consistent with UIP
   • Transbronchial lung biopsy generally not recommended for IPF evaluation
   • Lung cryobiopsy not routinely recommended

Disease Behavior Classification

The 2022 guidelines recommend classifying ILDs based on disease behavior patterns 2:

1. Reversible and self-limited (e.g., many cases of RB-ILD)
2. Reversible with risk of progression (e.g., cellular NSIP, DIP, COP)
3. Stable with residual disease (e.g., some fibrotic NSIP)
4. Progressive and irreversible with potential for stabilization (e.g., some fibrotic NSIP)
5. Progressive and irreversible despite therapy (e.g., IPF, some fibrotic NSIP)

Future Directions

The 2022 Chest guidelines highlight several areas for future research 1:

• Exploration of novel classification methods based on underlying biological features and anticipated treatment response
• Development of clearer terminology distinguishing radiologic/histopathologic patterns from clinical diagnoses
• Integration of genetic information in classification and management
• Investigation of biomarkers, molecular/genetic testing, and machine learning algorithms

Common Pitfalls in ILD Diagnosis

• Over-reliance on histopathology 2
• Inadequate exposure history 2
• Failure to recognize connective tissue disease 2
• Sampling error in lung biopsy 2
• Inconsistent application of diagnostic criteria 1

Management Implications

Treatment should be guided by specific ILD diagnosis 2:

• IPF: Antifibrotic therapy
• CTD-ILD: Immunomodulatory therapy
• HP: Antigen avoidance and immunosuppression
• Unclassifiable ILD: Treatment based on predominant disease behavior pattern

The classification of ILDs continues to evolve, with increasing emphasis on integrating multiple diagnostic modalities and considering disease behavior alongside morphologic features to optimize patient outcomes.