What is a lung infiltrate in the context of a patient with a computed tomography (CT) scan showing patchy multifocal infiltrate, possibly indicating an inflammatory or infectious process in an immunocompromised individual?

What is a Lung Infiltrate

A lung infiltrate is an abnormal substance or collection of material within the lung parenchyma that appears as increased density on imaging studies, representing fluid, cells, or other material filling the alveolar spaces or interstitial tissue. 1

Definition and Imaging Characteristics

Lung infiltrates represent pathological findings detected on chest imaging that indicate abnormal material accumulating in the lung tissue. 1 This material can include:

• Inflammatory cells (neutrophils, lymphocytes, eosinophils) in the alveolar septum or alveolar lumen 1
• Infectious organisms (bacteria, fungi, viruses) 1
• Fluid (edema, hemorrhage) 2
• Fibrotic tissue or granulation tissue 1
• Neoplastic cells 1, 2

Imaging Patterns in Immunocompromised Patients

In the context of patchy multifocal infiltrates on CT scan in an immunocompromised patient, several specific patterns help narrow the differential diagnosis:

Ground-Glass Opacities

• Diffuse bilateral perihilar infiltrates with patchy areas of ground-glass attenuation (with peripheral sparing), cysts, and septal thickening strongly suggest Pneumocystis pneumonia 1, 3
• Ground-glass attenuation can also indicate drug-related pneumonitis (such as from mycophenolate), viral infections, or acute inflammatory processes 3, 2

Nodular or Cavitary Lesions

• Nodular or cavitary lesions are highly suggestive of invasive filamentous fungal infection (Aspergillus, mucormycosis) 1, 3
• The "halo sign" (nodule surrounded by ground-glass opacity) and "air-crescent sign" are important CT findings indicating filamentous fungal disease 1
• The "reversed halo sign" (ground-glass opacity surrounded by consolidation ring) is relatively specific for fungal pneumonia due to zygomycetes/mucorales, though tuberculosis and other conditions remain in the differential 1

Consolidation

• Consolidation patterns can represent bacterial pneumonia, organizing pneumonia (BOOP), fungal infarction, or pulmonary hemorrhage 2

Critical Diagnostic Approach

High-resolution CT (HRCT) is essential and far superior to conventional chest radiographs, revealing pathological findings in approximately 50% of patients with normal chest X-rays 1, 3. In febrile neutropenic patients after >48 hours of antibiotics, only ~10% of chest radiographs show abnormalities while HRCT reveals pathology in ~50% of cases 1

Immediate Workup Required

• Obtain HRCT chest immediately to characterize the infiltrate pattern 4, 3
• Perform bronchoalveolar lavage (BAL) with comprehensive infectious workup including bacterial, fungal, viral cultures, galactomannan, β-D-glucan, and Pneumocystis PCR 3, 5, 6
• Two sets of blood cultures before initiating antibiotics 3, 5

Clinical Significance in Immunocompromised Hosts

The prognosis of lung infiltrates in immunocompromised patients is critical, requiring prompt diagnosis and treatment to reduce mortality 1, 6. The broad differential includes:

• Opportunistic infections: Pneumocystis jirovecii, invasive aspergillosis, mucormycosis, cytomegalovirus, mycobacteria, Nocardia 1, 3
• Drug-induced pneumonitis: Particularly with mycophenolate, chemotherapy agents 3, 5
• Underlying malignancy involvement 1
• Disease-specific complications: Acute lupus pneumonitis in SLE patients 5

Common Pitfall

Do not delay antimicrobial therapy while pursuing diagnostic procedures 1. In immunocompromised patients with fever and pulmonary infiltrates, initiate empiric broad-spectrum antibiotics immediately, add mold-active antifungal therapy if CT shows nodular/cavitary lesions, and start high-dose trimethoprim-sulfamethoxazole for Pneumocystis coverage in high-risk patients (CD4 <200, high-dose corticosteroids with other immunosuppressants) 4, 3, 5

Isolation of Aspergillus or other filamentous fungi from respiratory specimens in severely immunocompromised patients typically indicates true respiratory tract mycosis, not colonization 1