What is Fever?
Fever is an elevation of core body temperature above normal baseline, with the specific threshold varying by clinical context: most commonly defined as ≥38.3°C (101°F) for a single measurement in critically ill adults, or ≥38.0°C (100.4°F) for hospital-acquired infections. 1, 2
Core Definition and Temperature Thresholds
The definition of fever is not uniform and depends on the clinical population and setting:
- Adult ICU patients: A single temperature ≥38.3°C (101°F) is the standard definition recommended by the American College of Critical Care Medicine and Infectious Diseases Society of America 1, 2
- Hospital-acquired infections: The CDC defines fever as >38°C (100.4°F) 1, 2
- Neutropenic patients: Either a single oral temperature >38.3°C (101°F) OR ≥38.0°C (100.4°F) sustained over 1 hour, reflecting the lower threshold needed in immunocompromised hosts 1, 2
- Older adults in long-term care: A single oral temperature >37.8°C (100°F), or repeated measurements >37.2°C (99°F) orally or >37.5°C (99.5°F) rectally 1
- Pediatric patients: Temperature ≥38°C (100.4°F) 1
Physiological Context
Normal body temperature is traditionally considered 37.0°C (98.6°F), but this varies by 0.5-1.0°C due to circadian rhythm, menstrual cycle, and other factors 3. Importantly, evidence shows that normal human body temperature has been decreasing by 0.03°C per birth decade over the last 157 years 1.
Fever represents a physiological reset of the thermoregulatory set-point in the hypothalamus, typically triggered by exogenous microbial stimuli that activate phagocytes to release endogenous pyrogens (fever-inducing hormones). 4 This is distinct from hyperthermia, which represents a pathologic loss of thermoregulation 5.
Accurate Temperature Measurement
The hierarchy of measurement accuracy is critical for proper diagnosis:
Central monitoring methods are preferred: pulmonary artery catheter thermistors, bladder catheter thermistors, and esophageal balloon thermistors provide the most accurate core temperature readings 1, 2
When central monitoring is unavailable:
- Oral or rectal temperatures are acceptable alternatives 1, 2
- Rectal readings typically run a few tenths of a degree higher than core temperature 3
Avoid unreliable methods: Axillary measurements, tympanic membrane thermometers, temporal artery thermometers, and chemical dot thermometers should not be used for diagnostic purposes in critical care settings 1, 2
Critical Clinical Caveats
Absence of Fever Does Not Exclude Infection
A substantial proportion of infected patients are not febrile and may be euthermic or hypothermic, yet still have life-threatening infections. 3 This is particularly true in:
- Elderly patients 3
- Patients with open abdominal wounds or large burns 3, 2
- Patients receiving extracorporeal membrane oxygenation or continuous renal replacement therapy 3, 2
- Patients with congestive heart failure, end-stage liver disease, or chronic renal failure 3
- Patients taking anti-inflammatory or antipyretic drugs 3
Alternative Infection Indicators
When fever is absent but infection is suspected, look for 3, 2:
- Unexplained hypotension, tachycardia, or tachypnea
- New confusion or altered mental status
- Rigors or new skin lesions
- Oliguria or lactic acidosis
- Leukocytosis, leukopenia, or ≥10% immature neutrophils (bands)
- Thrombocytopenia
In older adults specifically, suspect infection with new or increasing confusion, incontinence, falling, deteriorating mobility, reduced food intake, or failure to cooperate with staff, even without fever. 1
Environmental and Iatrogenic Factors
Multiple ICU interventions can alter body temperature independent of infection, including specialized mattresses, hot lights, air conditioning, cardiopulmonary bypass, peritoneal lavage, dialysis, and continuous hemofiltration 3. These must be considered when interpreting temperature measurements.
Biological Purpose
Fever appears to have evolved as an adaptive mechanism for controlling infection 4. The restricted range of upper physiological temperatures (38-39°C) supports optimal activation of resting lymphocytes for immune response generation, while established effector mechanisms function independently of temperature 6. This suggests fever's biological purpose is eliminating lower peripheral tissue temperatures rather than simply elevating core temperature. 6