What is fever?

What is Fever?

Fever is an elevation of core body temperature above normal physiological range, defined as a single temperature measurement ≥38.3°C (101°F) in most adult clinical settings, though specific thresholds vary by patient population and clinical context. 1, 2

Core Definition and Temperature Thresholds

Fever represents a physiologic reset of the body's thermostatic set-point in response to exogenous or endogenous pyrogens, distinguishing it from hyperthermia where thermoregulation is lost. 3, 4 The specific temperature threshold depends on the clinical population:

Population-Specific Definitions

• Adult ICU patients: Single temperature ≥38.3°C (101°F) 1, 2
• Hospital-acquired infections: Temperature >38.0°C (100.4°F) 1
• Neutropenic patients: Single oral temperature >38.3°C (101°F), OR temperature ≥38.0°C (100.4°F) sustained over 1 hour 1, 2
• Older adults in long-term care: Single oral temperature >37.8°C (100°F), OR repeated oral temperatures >37.2°C (99°F), OR rectal temperatures >37.5°C (99.5°F) 1
• Pediatric patients: Temperature ≥38.0°C (100.4°F) 1, 2

Normal Body Temperature Context

Normal body temperature is traditionally considered 37.0°C (98.6°F), but this varies by 0.5-1.0°C based on circadian rhythm, menstrual cycle, and age. 5 Importantly, normal human body temperature has been decreasing by 0.03°C per birth decade over the last 157 years, making historical standards less applicable. 1

Pathophysiology

Fever is initiated when immune cells in the periphery interact with pyrogens (infectious or non-infectious triggers), producing pyrogenic cytokines and prostaglandins. 3 These signals reach the hypothalamus via two routes:

• Humoral pathway: Cytokines and prostaglandins from blood and liver immune cells transmit directly to the pre-optic area of hypothalamus through circumventricular organs 3
• Neuronal pathway: Same cytokines stimulate vagal sensory neurons indirectly 3

The hypothalamus then resets the body's temperature set-point upward, triggering heat-generating and heat-conserving mechanisms. 4

Critical Clinical Caveat: 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. 5, 2 High-risk populations for absent fever despite serious infection include:

• Elderly patients 5, 2
• Patients with open abdominal wounds or large burns 5, 2
• Those receiving extracorporeal membrane oxygenation or continuous renal replacement therapy 5, 2
• Patients with congestive heart failure, end-stage liver disease, or chronic renal failure 5
• Those taking anti-inflammatory or antipyretic drugs 5

In these patients, alternative infection indicators should prompt evaluation: unexplained hypotension, tachycardia, tachypnea, new confusion, rigors, oliguria, lactic acidosis, leukocytosis, leukopenia, ≥10% immature neutrophils (bands), or thrombocytopenia. 5, 2

Accurate Temperature Measurement

The hierarchy of measurement accuracy (descending order) is: 2

1. Intravascular thermistor (pulmonary artery catheter)
2. Esophageal thermistor
3. Bladder thermistor
4. Rectal thermometer
5. Oral thermometer

Axillary measurements, temporal artery estimates, and chemical dot thermometers should never be used in critical care settings due to unreliability. 2 For alert, cooperative patients without central monitoring, oral or rectal temperatures are acceptable alternatives. 5, 1

Fever vs. Hyperthermia

Fever differs fundamentally from hyperthermia: fever involves a controlled elevation of the temperature set-point with supportive thermoeffector activities, while hyperthermia represents uncontrolled temperature rise without set-point elevation. 4 Fever typically has an upper limit of 41°C in humans, rarely exceeded during natural fevers. 4