What is the definition of fever?

Definition of Fever

Fever is defined as a single oral temperature measurement of ≥38.3°C (101°F) or a temperature of ≥38.0°C (100.4°F) sustained over a 1-hour period. 1

Temperature Measurement Methods

The definition of fever depends on the method used to measure temperature, as different body sites yield different readings:

Gold Standard Methods

• Core temperature measurements are considered the most accurate and include:
  • Pulmonary artery catheter thermistors
  • Bladder catheter thermistors
  • Esophageal balloon thermistors 1

Recommended Alternative Methods

• For most patients without core temperature monitoring devices:
  • Oral temperature - safe, convenient for alert and cooperative patients
  • Rectal temperature - typically reads a few tenths of a degree higher than core temperature 1

Less Reliable Methods (Not Recommended)

• Axillary temperatures
• Tympanic membrane temperatures
• Temporal artery thermometers
• Chemical dot thermometers 1

Population-Specific Definitions

Fever definitions vary based on patient population and clinical context:

1. General ICU patients: ≥38.3°C (101°F) single measurement 1

2. Neutropenic patients:

   • Single oral temperature ≥38.3°C (101°F) OR
   • Temperature ≥38.0°C (100.4°F) sustained over 1 hour 1

3. Hospital-acquired infections (CDC definition):

   • 38°C (100.4°F) 1

4. Elderly in long-term care facilities:

   • Single oral temperature >37.8°C OR
   • Repeated measurements >37.2°C (oral) or >37.5°C (rectal) OR
   • Increase from baseline >1.1°C 1

5. Pediatric patients:

   • Rectal temperature >38°C (>100.4°F) 1

Clinical Considerations

Normal Temperature Variations

• Normal body temperature is generally considered to be 37.0°C (98.6°F)
• Varies by 0.5-1.0°C due to:
  • Circadian rhythm
  • Menstrual cycle
  • Physical activity (can increase by 2-3°C with heavy exercise) 1
• Evidence indicates normal body temperature has been decreasing in the human population by 0.03°C per birth decade over the last 157 years 1

Special Populations Without Fever Despite Infection

Certain patients may have serious infections without developing fever:

• Elderly patients
• Patients with open abdominal wounds or large burns
• Patients on extracorporeal membrane oxygenation or continuous renal replacement therapy
• Patients with heart failure, end-stage liver disease, or chronic renal failure
• Patients taking anti-inflammatory or antipyretic medications 1

Warning Signs in Absence of Fever

In patients without fever, the following may indicate serious infection:

• Unexplained hypotension
• Tachycardia or tachypnea
• Confusion or altered mental status
• Rigors
• Skin lesions
• Respiratory manifestations
• Oliguria
• Lactic acidosis
• Leukocytosis, leukopenia, or ≥10% bands
• Thrombocytopenia 1

Clinical Pitfalls to Avoid

1. Relying solely on temperature thresholds: The definition of fever is arbitrary and depends on the purpose for which it is defined 1. A simple threshold for fever definition is outdated 2.

2. Using inappropriate measurement methods: Avoid axillary temperatures as they may not accurately reflect core body temperature. Rectal temperature measurements should be avoided in neutropenic patients 1.

3. Ignoring infection without fever: Not all patients with infection manifest fever, and the absence of fever in patients with infection is associated with worse outcomes 1.

4. Failing to monitor temperature trends: Relative changes in individual temperature may be more meaningful than absolute values 2.

5. Delaying treatment in high-risk patients: For neutropenic patients with fever, empirical antibiotic therapy should be administered urgently (within 2 hours) after presentation 1.