Organising Pneumonia (OP) - Diagnostic Workup

1. Classification First - Is it COP or Secondary OP?

2. Clinical Presentation

• Demographics: Mean age 50-60 years, equal sex distribution, no strong smoking association
• Onset: Subacute (usually <2 months), often misdiagnosed as community-acquired pneumonia
• Symptoms: Flu-like illness - nonproductive cough, progressive exertional dyspnoea, fever, malaise, fatigue, weight loss, night sweats
• Examination: Focal inspiratory crackles; finger clubbing is rare
• Key clue: Failure to respond to antibiotics in a patient treated for "pneumonia" - Harrison's Principles of Internal Medicine 22E

3. Laboratory Investigations

Bloods

These serologies are clinically important - CTD-associated OP often requires prolonged therapy, distinguishing it from isolated COP. - Harrison's 22E

Microbiology

• Sputum culture + sensitivities
• Blood cultures if febrile
• Urinary Legionella and pneumococcal antigen
• Respiratory viral panel (including COVID-19, influenza)
• Consider Pneumocystis jirovecii (PCP) PCR if immunocompromised

4. Pulmonary Function Tests

5. Imaging

Chest X-Ray

• Patchy unilateral or bilateral alveolar opacities, often peripheral
• Normal lung volumes (distinguishes from pulmonary fibrosis)
• Small nodular opacities in 10-50% of cases
• Pleural effusion and cavities are uncommon

HRCT Chest (KEY investigation)

CT findings are often far more extensive than expected from the chest X-ray. Grainger & Allison's Diagnostic Radiology

6. Bronchoalveolar Lavage (BAL)

7. Lung Biopsy

When is biopsy needed?

• Atypical clinical/radiological features
• Poor response to corticosteroids
• Diagnostic uncertainty (e.g., single mass lesion mimicking malignancy)
• In immunocompromised hosts (e.g., post-HSCT), biopsy may be required earlier

Biopsy approaches (in order of invasiveness)

Histopathological features (Masson bodies)

• Intraluminal fibroblastic buds (Masson bodies) in respiratory bronchioles, alveolar ducts, and alveoli - the hallmark
• Foamy macrophages in alveolar spaces
• Type 2 pneumocyte hyperplasia
• Interstitial mononuclear inflammatory infiltrate
• Notably: the lung architecture is preserved (no fibrosis/honeycombing distinguishes from UIP/IPF)

8. Additional Investigations to Exclude Secondary Causes

9. Diagnostic Algorithm Summary

Subacute flu-like illness + consolidation not responding to antibiotics
         ↓
HRCT chest (first key investigation)
         ↓
Classical CT pattern (peripheral/subpleural consolidation ± reverse halo)?
    YES ──→ BAL + serology screen (CTD, ANCA, serum precipitins)
               ↓
            Exclude secondary causes → if none found → COP likely
               ↓
            Consider TBBx; if inconclusive → VATS biopsy for histology
    NO  ──→ Consider alternative ILD / malignancy / infection workup

10. Treatment (Brief)

• Oral corticosteroids (prednisolone 0.75-1 mg/kg/day) for 6 months: rapid, dramatic response is characteristic and itself supports the diagnosis
• Relapse rate is high (~50%) on steroid taper - often needs prolonged treatment
• Steroid-sparing agents for relapsing disease: azathioprine, mycophenolate mofetil, cyclophosphamide
• Clarithromycin 250-500 mg/day: an alternative for mild disease (FVC >80% predicted)
• Rituximab: emerging evidence in steroid-refractory OP (especially CTD-associated)
• Spontaneous remission occurs in a minority