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  Fever is a symptom, not a disease.

  
  
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  Fever should not be confused with hyperthermia. Temperatures higher than 41°C (105.8°F) are almost always due to hyperthermia and not fever.

  
  
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  Be thoughtful in your evaluation of fever to avoid misdiagnosing a serious bacterial illness as “just another viral syndrome.”

  
  
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  Provide empiric antibiotics early for moderate to severely ill patients with a possible infectious etiology. Give directed antibiotic treatment in the emergency department to patients with serious focal bacterial infections.

The human body temperature is controlled within a narrow range between 36 and 37.8 °C (96.8 –100.4°F). Fever is defined as a core temperature >38° C (100.4 °F) in infants and >38.3°C (100.9°F) in adults. It is the result of the body resetting the temperature control center, the hypothalamus, in response to infection. Endogenous (cytokines) and exogenous (bacterial and viral) pyrogens trigger production of prostaglandin E2 (PGE2) in the hypothalamus. PGE2 raises the hypothalamic temperature set point. The body then generates and conserves heat to reach this new hypothalamic set point, thereby raising the body temperature. Fever is sustained as long as the levels of pyrogens and PGE2 are elevated. Cyclooxygenase inhibitors decrease fever by blocking the production of PGE2.

Fever is one of the most common presenting complaints in the emergency department (ED). It accounts for 5% of adult visits, 15% of elderly visits and 40% of pediatric visits to the ED. The most important thing to recognize about fever is that it is a symptom, not a disease, and it represents an underlying problem that must be evaluated and treated. The most common sites of infection vary based on age and immune system status. In the elderly and immunosuppressed, respiratory, genitourinary, and bacterial skin infections predominate. In younger patients the cause of fever is often self-limited and benign (eg, upper respiratory infection), but serious focal bacterial infections (eg, meningitis) requiring antibiotics, diagnostic procedures, and admission, must be detected.

History

The differential diagnosis for fever is quite broad, but in 85% of cases the cause is identified by a thorough history and physical examination. Important historical information includes the onset, magnitude, duration, pattern, any associated symptoms, travel within the past year, chronic illnesses, recent medication changes, recent hospitalizations, chemotherapy, radiotherapy, or the presence of indwelling vascular access devices or artificial heart valves. The age and overall health of the patient must be taken into account when taking the history and making medical decisions.

Physical Examination

The site of temperature recording should be noted, as rectal temperatures are more accurate and usually 1°C higher than oral temperatures. Rectal temperature should be taken in infants, children, and adults with significant tachypnea, tachycardia, or altered mental status (AMS). Heart rate (HR) and respiratory rate (RR) increase as fever rises. An increase in temperature of 1°C results in an increase in HR by approximately 10 bpm. The RR may also increase 2-4 breaths/minute per degree Celsius. The elderly and immunosuppressed patients may not mount a febrile response despite serious infection.

In most patients, the examination is directed by the patient’s symptoms (Table 33-1). Patients with significant alterations in mental status, respiratory distress, and cardiovascular instability require rapid assessment and stabilization. Once the patient has been stabilized, assess for infectious causes that may be a threat to life (eg, toxic shock, septic shock, meningitis, peritonitis).