Pulmonary Infiltrates in the Immunocompromised Host

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

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

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

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