Approach to Fever

Definitions and terminology of fever

Term Definition and characteristics 
Fever 
  • Elevated core body temperature above daily range for an individual regulated by hypothalamus 
  • Usually due to increased prostaglandin E2 (PGE2) 
  • Mediated by cytokines (eg, IL-1, IL-6, TNF) and the enzyme cyclooxygenase 
  • Temperature > 38.3o C (101o F) 
Hyperthermia 
  • Elevated body temperature rising in an uncontrolled manner that overrides the body’s ability to lose heat 
  • Can be due to exogenous heat exposure (eg, heat exhaustion) or endogenous heat production (eg, malignant hyperthermia) 
Hyperpyrexia 
  • Extraordinarily high fever (> 41.5o C) 
  • Can be due to severe infections or CNS hemorrhage 
Pyrogens 
  • Any substance that causes fever 
  • Exogenous pyrogen  
    • Derived from outside of the host 
    • Includes microbes or their toxins  
    • Example includes lipopolysaccharide endotoxin from gram-negative bacteria 
  • Pyrogenic cytokines 
    • Small proteins that regulate immune, inflammatory, and hematopoietic processes 
    • Examples include IL-1, IL-6, and TNF 
Fever of Unknown Origin (FUO) 
  • Fever > 38.3 O on several occasions for > 3 weeks in non-hospital setting 
  • Duration of fever > 1 week in hospitalized setting 
Neutropenic fever 
  • Single temperature > 38.3O C or sustained temperature > 38O C (100.4o F) 
  • Neutropenia 
    • Absolute Neutrophil Count (ANC) < 1500 cells/L 
    • Severe neutropenia is ANC < 500 cells/L 
(Next Lesson) Differential diagnosis of fever
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