What is the definition of fever?

Definition of Fever

Fever is defined as a single temperature measurement greater than or equal to 38.3°C (101°F), according to the Society of Critical Care Medicine and the Infectious Diseases Society of America. 1

Temperature Measurement Methods

The accuracy of fever detection depends significantly on the measurement method used:

Preferred Methods (in order of accuracy)

1. Central temperature monitoring methods:
   • Pulmonary artery catheter thermistors
   • Bladder catheter thermistors
   • Esophageal balloon thermistors

These are considered the gold standard for measuring core body temperature 1, 2.

1. Secondary methods (when central monitoring is unavailable):
   • Oral temperature measurement (for alert, cooperative patients)
   • Rectal temperature measurement

Methods to Avoid

• Axillary measurements (consistently lower than core temperature)
• Tympanic membrane infrared devices
• Chemical dot thermometers
• Noninvasive temporal artery thermometers

These methods have significant limitations and variability 1, 2.

Population-Specific Fever Definitions

Different clinical contexts require different fever thresholds:

• ICU patients: ≥38.3°C (101°F) 1
• Hospital-acquired infections: >38°C (100.4°F) 1, 2
• Neutropenic patients: Single oral temperature ≥38.3°C (101°F) or sustained temperature ≥38.0°C (100.4°F) for at least 1 hour 2
• Elderly in long-term care:
  • Single oral temperature >37.8°C (100.0°F)
  • Repeated oral measurements >37.2°C (99.0°F)
  • Repeated rectal measurements >37.5°C (99.5°F)
  • Increase from baseline >1.1°C (2.0°F) 1, 2

Normal Temperature Range

The normal body temperature range is subject to various factors including:

• Age
• Gender
• Diurnal variation
• Sampling site 1

Normal body temperature typically ranges from 36.5°C to 37.5°C (97.7°F to 99.5°F), with variations due to circadian rhythm and menstrual cycle 2.

Clinical Considerations

Important Caveats

1. Not all infected patients present with fever:

   • Elderly patients
   • Patients with open abdominal wounds
   • Patients with large burns
   • Patients on certain medical therapies
   • Patients receiving extracorporeal membrane oxygenation or continuous renal replacement therapy 1, 2

2. Absence of fever in infected patients is associated with worse outcomes 1, 2

3. Environmental factors in ICU settings can alter temperature readings 1, 2

4. Normal body temperature has been decreasing in the human population by 0.03°C per birth decade over the last 157 years 1

Clinical Interpretation

Temperature readings should be interpreted in context with:

• Individual's baseline temperature
• Presence of other symptoms
• Recent environmental conditions
• Changes in mental status or functional decline 2

Pitfalls to Avoid

1. Relying solely on temperature for diagnosis - Other signs like altered mental status, rigors, tachycardia, and decreased urine output may indicate infection even without fever 2

2. Using inappropriate measurement methods - Different methods yield different results, with variations of 0.5-1.0°C between axillary and oral temperatures, and 1.15°C between axillary and rectal temperatures 2

3. Applying a single definition across all populations - Different patient populations require different fever thresholds 1, 2

4. Overlooking fever in the absence of other symptoms - Fever alone may be the only presenting sign of serious infection in some patients 1

By understanding these nuances in fever definition and measurement, clinicians can more accurately identify and respond to potentially serious infections across different patient populations.