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 Thresholds by Patient Population
Different clinical contexts require different definitions of fever:
- General adult patients: Core temperature greater than 38.0°C (100.4°F) 1
- ICU patients: Temperature ≥38.3°C (101°F) 1
- Neutropenic patients: Single oral temperature >38.3°C (101°F) or >38.0°C (100.4°F) sustained over 1 hour 1
- Elderly in long-term care: Single oral temperature >37.8°C (100°F), repeated measurements >37.2°C (oral) or >37.5°C (rectal), or increase from baseline >1.1°C 1
- Pediatric patients: Rectal temperature >38°C (>100.4°F) 1
Measurement Methods and Accuracy
Temperature measurement methods vary in reliability:
Gold standard methods (in order of preference):
- Pulmonary artery catheter thermistors
- Bladder catheter thermistors
- Esophageal balloon thermistors 1
Alternative methods (when invasive monitoring is not available):
- Oral temperature (safe and convenient for alert patients)
- Rectal temperature (reliable but has risks of trauma, perforation, and spreading pathogens) 1
Less reliable methods (not recommended when accuracy is critical):
- Axillary temperature (consistently lower than core temperature)
- Tympanic membrane temperature
- Temporal artery thermometers
- Chemical dot thermometers 1
Clinical Significance and Interpretation
The normal body temperature range is affected by multiple factors:
- Age, gender, diurnal variation, and sampling site 1
- Normal body temperature has been decreasing in the human population by 0.03°C per birth decade over the last 157 years 1
- Normal body temperature is generally considered to be 37.0°C (98.6°F), with variations of 0.5-1.0°C due to circadian rhythm and menstrual cycle 1
Important Clinical Considerations
- Not all infected patients manifest fever; absence of fever in patients with infection is associated with worse outcomes 1, 2
- The definition of fever is somewhat arbitrary and depends on the purpose for which it is defined 1
- In neutropenic patients, fever represents an emergency requiring urgent antibiotic treatment 1, 2
- Fever represents a normal physiologic response that may result from the introduction of an infectious pathogen and is hypothesized to play a role in fighting infections 1
Pitfalls to Avoid
- Relying on axillary temperatures alone, which may not accurately reflect core body temperature 1, 2
- Assuming all fevers indicate serious infection; most fevers are caused by benign conditions 3
- Failing to recognize that absence of fever doesn't rule out serious infection, especially in elderly or immunocompromised patients 1, 2
- Using a single temperature threshold for all patient populations rather than adjusting based on clinical context 1
- Treating uncomplicated fever unnecessarily; antipyretics may mask important diagnostic signs 3
The definition of fever should be applied in the context of the clinical situation, with consideration of the patient's baseline temperature, measurement method, and overall clinical presentation.