What is the definition of fever in adults and children?

Definition of Fever

Fever is defined as a core body temperature ≥38.3°C (101°F) for a single measurement in adults, or ≥38.0°C (100.4°F) in children, measured rectally. 1, 2

Adult Fever Definitions

The threshold varies based on clinical context and measurement method:

• Standard adult definition: A single core temperature measurement ≥38.3°C (101°F) is the most widely accepted threshold per the American College of Critical Care Medicine and Infectious Diseases Society of America 1, 2

• Alternative adult definition: Some sources define fever as ≥38.0°C (100.4°F), particularly for hospital-acquired infections per CDC guidelines 2

• Two-measurement criterion: Two consecutive temperature elevations of 38.3°C (101°F) is an acceptable alternative definition 1

• Neutropenic patients: Fever is defined as either a single oral temperature >38.3°C (101°F) OR a sustained temperature ≥38.0°C (100.4°F) for 1 hour 1, 2

• Older adults in long-term care: The Infectious Diseases Society of America recommends a lower threshold—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) 2

Pediatric Fever Definitions

Children require rectal temperature measurement for accuracy:

• Standard pediatric definition: Rectal temperature ≥38°C (100.4°F) per the American Academy of Pediatrics 1, 2

• Age consideration: This definition applies to children aged 1 day to 3 years 1

• Measurement reliability: Rectal temperature is the gold standard in young children; other methods (oral, axillary, tympanic) have lower reliability and must be interpreted cautiously 1

Temperature Measurement Methods

Accuracy depends critically on measurement site:

• Most accurate (core temperature): Pulmonary artery catheter thermistors are the gold standard, followed by bladder catheter thermistors and esophageal probes in the distal third of the esophagus 1

• Practical alternatives: When central monitoring is unavailable, rectal temperatures (often a few tenths of a degree higher than core) or oral temperatures are preferred 1, 2

• Pediatric measurement: Rectal temperature is recommended for neonates and young children; oral temperature for older cooperative children 2

• Avoid less reliable methods: Axillary and tympanic measurements are significantly less accurate and should be avoided when clinical decisions depend on precise temperature 2

Normal Temperature Variations

Understanding baseline temperature is essential to avoid misdiagnosis:

• Traditional baseline: Normal body temperature is generally considered 37.0°C (98.6°F), though this varies by 0.5-1.0°C due to circadian rhythm and menstrual cycle 1

• Evolving baseline: Evidence shows human body temperature has decreased by 0.03°C per birth decade over the last 157 years 3, 2

• Physiologic increases: Heavy exercise can raise temperature by 2-3°C without representing pathology 1

• Environmental factors: ICU patients face multiple temperature-altering factors including specialized mattresses, hot lights, air conditioning, cardiopulmonary bypass, dialysis, and continuous hemofiltration 1

Critical Clinical Caveat: Absence of Fever Does Not Exclude Serious Infection

Many infected patients never develop fever, and this carries worse prognosis:

• High-risk populations for absent fever response: Elderly patients, those with open abdominal wounds, large burns, patients receiving extracorporeal membrane oxygenation or continuous renal replacement therapy, and those with congestive heart failure, end-stage liver disease, chronic renal failure, or taking anti-inflammatory/antipyretic drugs 1, 3

• Alternative infection indicators: In euthermic or hypothermic patients, search for unexplained hypotension, tachycardia, tachypnea, confusion, rigors, skin lesions, respiratory manifestations, oliguria, lactic acidosis, leukocytosis, leukopenia, immature neutrophils ≥10%, or thrombocytopenia 1, 3

• Prognostic significance: The absence of fever in infected patients is associated with worse outcomes and should trigger aggressive evaluation and empirical therapy based on clinical presentation 1, 3

• Geriatric considerations: In older adults, suspect infection with new confusion, incontinence, falling, deteriorating mobility, reduced food intake, or failure to cooperate with staff, even without fever 2

Common Pitfalls to Avoid

• Inconsistent measurement methods: Using different temperature measurement sites yields significantly different readings; maintain consistency within individual patients 2

• Age-specific definition failure: Applying adult fever thresholds to elderly patients (who have lower baseline temperatures) or using inappropriate thresholds for long-term care residents leads to missed infections 2

• Over-reliance on temperature alone: Failing to consider non-temperature signs of infection, particularly functional decline in older adults, results in delayed diagnosis 2

• Ignoring measurement site: The term "fever" is meaningless without specifying the measurement site and method used 4