What is considered a fever in adults and children?

What is Considered a Fever?

Fever is defined as a single temperature measurement ≥38.3°C (101°F) in adults and ≥38.0°C (100.4°F) in children, though specific thresholds vary by clinical population and setting. 1, 2

Standard Fever Definitions by Population

Adults

• ICU/hospitalized adults: Single temperature ≥38.3°C (101°F) per Society of Critical Care Medicine and Infectious Diseases Society of America 1, 2
• Hospital-acquired infections: ≥38.0°C (100.4°F) per CDC definition 1, 2
• Elderly in long-term care facilities: Single oral temperature ≥37.8°C (100°F), OR repeated measurements ≥37.2°C (oral) or ≥37.5°C (rectal), OR increase from baseline >1.1°C 1, 2

Children

• Children <3 years: Rectal temperature ≥38.0°C (100.4°F) 1, 2
• Infants 1-3 months: Same threshold of ≥38.0°C (100.4°F) rectal 1

Special Populations

• Neutropenic/immunocompromised patients: Single oral temperature ≥38.3°C (101°F) OR sustained temperature ≥38.0°C (100.4°F) for ≥1 hour 1, 2
• CAR T-cell therapy patients: Fever ≥38.0°C defines Grade 1 cytokine release syndrome 2

Temperature Measurement Methods (Accuracy Hierarchy)

Use the most accurate method available, as measurement site significantly affects readings. 1, 2

Most Accurate (Gold Standard)

• Intravascular thermistor (pulmonary artery catheter) 1, 2
• Esophageal thermistor 1, 2
• Bladder thermistor 1, 2

Acceptable Alternatives

• Rectal thermometer (may differ by few tenths of degree from core) 1, 2
• Oral thermometer 1, 2

Unreliable Methods (Avoid in Critical Settings)

• Axillary measurements 1, 2
• Tympanic membrane thermometers 1, 2
• Temporal artery thermometers 1, 2
• Chemical dot thermometers 1, 2

Critical Clinical Pitfalls

Absence of Fever Does NOT Exclude Serious Infection

Many high-risk patients with life-threatening infections remain afebrile or hypothermic, and this paradoxically predicts worse outcomes. 1, 3

High-risk populations with blunted fever responses include: 3

• Elderly patients
• Immunosuppressed patients
• Patients on corticosteroids or anti-inflammatory medications
• End-stage liver disease, chronic renal failure, or congestive heart failure patients
• Patients receiving continuous renal replacement therapy or extracorporeal membrane oxygenation
• Patients with large burns or open abdominal wounds

Alternative infection indicators when fever is absent: 2, 3

• 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)
• Elevated procalcitonin or C-reactive protein

Temperature Variations Affect Interpretation

• Normal body temperature varies by age, gender, time of day, and measurement site 1, 3
• Human population average body temperature has decreased by 0.03°C per birth decade over the last 157 years 1, 3
• Antipyretic use within 4 hours may mask fever at presentation 1

High-Risk Fever Thresholds

Certain populations face increased mortality at lower fever thresholds: 4

• Neurologic injury patients (stroke, intracerebral hemorrhage): Even 1°F elevation above 99.6°F independently predicts poor outcomes and increased mortality 4
• Critically ill ICU patients: Mortality increases with fever >38.3°C (101°F), though absence of fever in infected patients paradoxically indicates worse prognosis 4