Pulmonary Infiltrates in the Immunocompromised Host

Key Features

• Immunocompromised patients defined as those with

  • HIV disease

  • Absolute neutrophil counts < 1000/mcL (< 1.0 × 10⁹/L)

  • Current or recent exposure to myelosuppressive or immunosuppressive drugs, or those currently taking > 5 mg/day of prednisone

• May arise from infectious or noninfectious causes

  • Infectious: bacterial, mycobacterial, fungal, protozoal, helminthic, or viral pathogens

  • Noninfectious processes: pulmonary edema, alveolar hemorrhage, drug reactions, pulmonary thromboembolic disease, malignancy, and radiation pneumonitis may mimic infection

Clinical Findings

• Although almost any pathogen can cause pneumonia in an immunocompromised host, two clinical tools help the clinician narrow the differential diagnosis

  • The first is knowledge of the underlying immunologic defect

    • Defects in humoral immunity predispose to bacterial infections

    • Defects in cellular immunity lead to infections with viruses, fungi, mycobacteria, and protozoa

    • Neutropenia and impaired granulocyte function predispose to infections from Staphylococcus aureus, Aspergillus, gram-negative bacilli, and Candida

  • Second, the time course of infection also provides clues to the etiology of pneumonia in immunocompromised patients

    • A fulminant pneumonia is often caused by bacterial infection

    • An insidious pneumonia is more apt to be caused by viral, fungal, protozoal, or mycobacterial infection

    • Pneumonia occurring within 2–4 weeks after organ transplantation is usually bacterial, whereas several months or more after transplantation P jirovecii, viruses (eg, cytomegalovirus), and fungi (eg, Aspergillus) are encountered more often

Diagnosis

• Chest radiography is rarely helpful in narrowing the differential diagnosis

• Examination of expectorated sputum for bacteria, fungi, mycobacteria, Legionella, and P jirovecii is important and may preclude the need for expensive, invasive diagnostic procedures

• Bronchoalveolar lavage using the flexible bronchoscope

  • Safe and effective method for obtaining representative pulmonary secretions for microbiologic studies

  • Involves less risk of bleeding and other complications than bronchial brushing and transbronchial biopsy

  • Especially suitable for the diagnosis of P jirovecii pneumonia in patients with AIDS when induced sputum analysis is negative

• Surgical lung biopsy, now often performed by video-assisted thoracoscopy, provides the definitive option for diagnosis

Treatment

• Routine evaluation frequently fails to identify a causative organism

• Empiric antimicrobial therapy may be started before proceeding to invasive procedures such as bronchoscopy, transthoracic needle aspiration, or open lung biopsy

• The approach to management must be based on the severity of the pulmonary infection, the underlying disease, the risks of empiric therapy, and local expertise and experience with diagnostic procedures