What is the definition of fever?

Definition of Fever

Fever in adult ICU patients is defined as a single temperature measurement ≥38.3°C (101°F), according to the most recent 2023 Society of Critical Care Medicine and Infectious Diseases Society of America guidelines. 1

Population-Specific Definitions

The definition of fever varies by clinical population and context:

Adult Populations

• ICU patients: Single temperature ≥38.3°C (101°F) 1, 2, 3
• Hospital-acquired infections (CDC): Temperature >38.0°C (100.4°F) 1, 2, 3
• Elderly in long-term care facilities: 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), OR increase from baseline >1.1°C 1, 3

Immunocompromised Patients

• Neutropenic patients: Single oral temperature ≥38.3°C (101°F) OR sustained temperature ≥38.0°C (100.4°F) for ≥1 hour 1, 2, 3

Pediatric Populations

• Children <3 years: Rectal temperature ≥38.0°C (100.4°F) 1, 2, 3

Temperature Measurement Hierarchy

Accuracy matters significantly—different measurement sites yield different values. 1

Most Accurate Methods (in descending order):

• Pulmonary artery catheter thermistor (gold standard when available) 1
• Bladder catheter thermistor (provides continuous readings identical to intravascular sites) 1
• Esophageal thermistor (distal third placement) 1
• Rectal temperature (often reads 0.1-0.3°C higher than core) 1
• Oral temperature (safe and convenient for alert patients) 1

Methods to Avoid:

• Do not use axillary measurements, temporal artery estimates, or chemical dot thermometers in critical care settings 2

Critical Clinical Context

Normal Temperature Variations

• Normal body temperature is generally 37.0°C (98.6°F) but varies by 0.5-1.0°C based on circadian rhythm and menstrual cycle 1
• Evidence shows human body temperature has decreased by 0.03°C per birth decade over the past 157 years 1, 3, 4
• Heavy exercise can raise temperature by 2-3°C 1

When Fever is Absent Despite Infection

A substantial proportion of infected patients are not febrile—absence of fever does not exclude serious infection and is associated with worse outcomes. 1, 4

High-risk populations for absent fever response include:

• Elderly patients 1, 4
• Patients with open abdominal wounds or large burns 1, 2, 4
• Patients receiving extracorporeal membrane oxygenation or continuous renal replacement therapy 1, 2, 4
• Patients with congestive heart failure, end-stage liver disease, or chronic renal failure 1, 4
• Patients taking anti-inflammatory or antipyretic drugs 1, 4

Alternative Infection Indicators (when fever absent):

• Unexplained hypotension, tachycardia, or tachypnea 1, 2, 4
• New confusion or altered mental status 1, 2
• Rigors or new skin lesions 1, 2
• Oliguria or lactic acidosis 1, 2
• Leukocytosis, leukopenia, or ≥10% immature neutrophils (bands) 1, 2
• Thrombocytopenia 1, 2

Common Pitfalls

The definition of fever is arbitrary and depends on clinical purpose—there is no single universal threshold. 1, 5, 6 When reviewing literature or implementing protocols, always verify:

• The specific temperature cutoff used 5, 6
• The measurement site 5, 6
• The type of thermometer employed 5, 6

Environmental factors in ICU settings can alter temperature measurements, including specialized mattresses, hot lights, air conditioning, cardiopulmonary bypass, peritoneal lavage, dialysis, and continuous hemofiltration. 1