1. Overview and Definitions

Febrile neutropenia (FN) is a common and serious complication of myelosuppressive chemotherapy, defined by the presence of fever in the setting of a critically low absolute neutrophil count (ANC). It represents a medical emergency due to the high risk of rapid progression to severe sepsis and death from an otherwise occult infection.

A. Diagnostic Criteria

• Fever: A single oral temperature of ≥ 38.3 °C (101 °F) or a sustained temperature of ≥ 38.0 °C (100.4 °F) for at least one hour. Note that hypothermia (< 36.0 °C) can also be a sign of severe infection, particularly in profoundly immunosuppressed patients.
• Neutropenia: An ANC below 500 cells/mm³, or an ANC expected to fall below 500 cells/mm³ within 48 hours. The severity is further stratified:
  • Mild Neutropenia: ANC 500–1,000 cells/mm³
  • Severe Neutropenia: ANC < 500 cells/mm³
  • Profound Neutropenia: ANC < 100 cells/mm³

B. Clinical Significance

FN is a major cause of morbidity, mortality, and healthcare costs in oncology. It complicates 10–50% of treatment courses for solid tumors and up to 80% for hematologic malignancies. Hospitalization is required in 20–30% of cases, and the infection-related mortality rate can be as high as 11%, even with modern supportive care.

2. Epidemiology and Incidence

The incidence of FN is highly dependent on the cancer type, the intensity of the chemotherapy regimen, and patient-specific factors. Understanding these variables is key to risk stratification and the appropriate use of prophylactic measures.

Morbidity and Healthcare Burden

FN significantly impacts patient outcomes and healthcare resources. Approximately 5–10% of hospitalized FN patients require ICU admission. In hematologic malignancies, FN contributes to nearly 50% of all infection-related deaths. Each episode is associated with prolonged hospital stays, increased costs, and potential delays or dose reductions in subsequent chemotherapy cycles, which can compromise treatment efficacy.

3. Pathophysiology of Immunosuppression

FN results from a “two-hit” process: profound neutropenia combined with damage to mucosal barriers. This creates a perfect storm where the body’s primary defense against microbes is eliminated just as endogenous flora are given a direct path into the bloodstream.

Mechanisms of Immune Failure

• Neutrophil Dysfunction: Chemotherapy not only reduces the number of neutrophils but also impairs the function of any remaining cells. This includes decreased chemotaxis (ability to move to the site of infection), phagocytosis (engulfing pathogens), and formation of neutrophil extracellular traps (NETs).
• Mucosal Barrier Injury (MBI): The rapidly dividing cells of the gastrointestinal mucosa are highly susceptible to chemotherapy, leading to mucositis. This breakdown of the physical barrier allows gut flora, such as Pseudomonas aeruginosa, Enterobacteriaceae, and Candida species, to invade the systemic circulation.
• Cytokine Dysregulation: The inflammatory response is blunted. Reduced levels of key signaling molecules like IL-8 and endogenous G-CSF impair the recruitment and proliferation of neutrophils, while increased levels of anti-inflammatory cytokines like IL-10 can mask classic signs of infection, leading to atypical presentations.

4. Impact of Chronic Diseases and Special Populations

Pre-existing comorbidities and specific patient populations significantly alter the risk profile, the likely pathogens, and the clinical course of FN.

A. Hematologic Malignancies & Stem Cell Transplant (HSCT)

This is the highest-risk population due to the intensity of therapy, prolonged periods of profound neutropenia, and concurrent immunosuppression from graft-versus-host disease (GVHD) prophylaxis. These patients are uniquely vulnerable to multidrug-resistant (MDR) bacteria and invasive molds like Aspergillus and Mucorales.

B. HIV/AIDS-Associated Neutropenia

Patients with HIV face a dual defect: chemotherapy-induced neutropenia is compounded by underlying T-cell depletion and viral effects on bone marrow function. Empiric antimicrobial coverage may need to be broadened to include opportunistic pathogens like Mycobacterium avium complex (MAC) or Cryptococcus.

C. Other Key Comorbidities

• Diabetes Mellitus: Impairs neutrophil chemotaxis and phagocytosis, increasing susceptibility to infection.
• Chronic Kidney Disease (CKD): Complicates management by altering the clearance of many antibiotics, requiring careful dose adjustments.
• Liver Disease: Reduces the synthesis of acute-phase reactants and clotting factors, potentially masking inflammatory markers and increasing bleeding risk.

5. Social Determinants of Risk

Clinical factors alone do not determine FN risk. Social determinants of health (SDOH) play a critical role in patient outcomes by influencing access to care, adherence to treatment, and the ability to recognize and report symptoms early.

A. Medication Access and Adherence

Financial toxicity is a major barrier. The high cost of G-CSF and prophylactic antibiotics can lead to non-adherence. Patients may skip doses or fail to fill prescriptions, directly increasing their risk of developing FN.

B. Health Literacy and Early Presentation

A patient’s ability to understand their condition and treatment plan is crucial. Low health literacy can lead to a failure to recognize fever as an emergency or a delay in seeking care. Structured educational interventions that teach patients how to take their temperature and when to call the clinic have been shown to shorten the critical “time-to-antibiotics.”

C. Socioeconomic and Logistical Barriers

Factors such as lack of reliable transportation, unstable housing, food insecurity, and social isolation can severely impede a patient’s ability to attend appointments, obtain medications, and maintain the hygiene and nutrition necessary to reduce infection risk.

6. Translating Foundational Knowledge into Practice

Effective management of FN relies on translating our understanding of its epidemiology and pathophysiology into actionable, personalized strategies for risk assessment and prevention.

A. Risk Assessment Algorithms

Validated scoring systems help clinicians stratify patients to determine the appropriate site of care (inpatient vs. outpatient) and intensity of treatment.

B. Preventive Measures and Education

Proactive strategies are the cornerstone of reducing the burden of FN:

• G-CSF Prophylaxis: For all high-risk regimens (>20% FN risk) and for intermediate-risk regimens in patients with additional risk factors.
• Antibiotic Prophylaxis: Fluoroquinolone prophylaxis (e.g., levofloxacin) is considered for patients expected to have prolonged, profound neutropenia (>7 days), though this must be balanced against the risk of promoting resistance.
• Patient Education: Structured, repeated education on fever recognition, proper use of a thermometer, and the urgent need to contact the oncology team is paramount.

C. Future Directions

The field is moving toward more precise, personalized risk models. Emerging biomarkers like procalcitonin and presepsin may help distinguish bacterial infection from other causes of fever earlier. In the future, machine-learning algorithms that integrate clinical, genomic, and social determinant data may offer highly accurate, real-time risk prediction to guide prophylactic interventions.