What temperature defines fever in adults and children, and what are the recommended treatments and red‑flag signs requiring urgent evaluation?

Fever Temperature Definition and Measurement

Temperature Thresholds Defining Fever

Fever in adults is defined as a single temperature ≥38.3°C (101°F), while in children younger than 3 years fever is defined as a rectal temperature ≥38.0°C (100.4°F). 1

Adults

• Critical care/hospitalized adults: ≥38.3°C (101°F) single measurement 1, 2
• Hospital-acquired infection surveillance: ≥38.0°C (100.4°F) 2
• 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

Elderly (Long-Term Care Facilities)

• Single oral temperature >37.8°C (100°F) [70% sensitive, 90% specific for infection] 2, 3
• Alternative criteria: Repeated oral >37.2°C (99°F) or rectal >37.5°C (99.5°F) 2
• Baseline change: Increase >1.1°C from individual baseline 2

Children

• Age <30 days: ≥38.0°C (100.4°F) rectal 1, 4, 5
• Age 31-60 days: ≥38.1°C (100.4°F) rectal 5
• Age 61-91 days: ≥38.2°C (100.4°F) rectal 5
• Age ≥4 years: ≥38.0°C (100.4°F) oral if cooperative 4

Recommended Temperature Measurement Methods

Use central monitoring (pulmonary artery, bladder, or esophageal thermistors) when available in critically ill patients; otherwise use rectal or oral thermometry—never axillary, tympanic, or temporal artery devices in acute care settings. 1, 2

Accuracy Hierarchy (Most to Least Accurate)

1. Pulmonary artery catheter thermistor (gold standard) 1, 2
2. Bladder catheter thermistor (continuous, intravascular-equivalent) 2
3. Esophageal thermistor (distal third placement) 2
4. Rectal thermometer (reads 0.2-0.3°C higher than core) 1, 2
5. Oral thermometer (safe for alert, cooperative patients) 1, 2

Age-Specific Measurement Recommendations

• Infants and children <4 years: Rectal temperature is gold standard 1, 4
• Children ≥4 years: Oral temperature acceptable if cooperative 4
• Alert adults: Oral temperature convenient and safe 1
• Critically ill/intubated patients: Use central monitoring or rectal if central unavailable 1, 2

Methods to Avoid

• Never use in acute care: Axillary measurements, tympanic/infrared ear devices, temporal artery scanners, chemical dot thermometers 1, 2, 3
• These methods are unreliable and should not guide clinical decisions 1, 2

Rectal Thermometry Contraindications

• Neutropenic patients 1
• Coagulopathy or thrombocytopenia 1
• Recent rectal surgery 1
• Risk of spreading enteric pathogens (C. difficile, VRE) 1

Red-Flag Signs Requiring Urgent Evaluation

Absence of fever does not exclude serious infection—a substantial proportion of infected patients remain euthermic or hypothermic, and this is associated with worse outcomes. 1, 2

High-Risk Populations with Blunted Fever Response

• Elderly patients 1, 2
• Large burns or open abdominal wounds 1, 2
• ECMO or continuous renal replacement therapy 1, 2
• Congestive heart failure, end-stage liver disease, chronic renal failure 1, 2
• Anti-inflammatory drugs, corticosteroids, or antipyretics 1, 2

Alternative Infection Indicators (When Fever Absent)

Initiate comprehensive infection work-up if any present:

• Hemodynamic: Unexplained hypotension, tachycardia, tachypnea 1, 2
• Neurologic: New confusion, altered mental status 1, 2, 3
• Physical signs: Rigors, new skin lesions 1, 2
• Renal/metabolic: Oliguria, rising lactate 1, 2
• Hematologic: Leukocytosis, leukopenia, ≥10% immature neutrophils (bands), thrombocytopenia 1, 2
• Geriatric-specific: New incontinence, falls, deteriorating mobility, reduced food intake, failure to cooperate 3

Pediatric Red Flags

• All neonates (0-28 days) with fever ≥38.0°C: Require immediate hospitalization with full sepsis evaluation (13% risk of severe bacterial infection) 4
• Young infants (29-90 days): 9% incidence of severe bacterial infection; require careful stratification 4
• Critical pitfall: Only 58% of infants with bacteremia or bacterial meningitis appear clinically ill 4
• Recent antipyretic use (within 4 hours): Can mask fever and severe infection 4
• Viral infection present: Does not exclude bacterial co-infection 4

Treatment Considerations

When to Initiate Sepsis Work-Up

• Neutropenic/critically ill adults: Fever ≥38.3°C or sustained ≥38.0°C for 1 hour warrants blood cultures, imaging, and empirical broad-spectrum antibiotics 1, 2
• Neonates: Any fever ≥38.0°C requires full sepsis evaluation regardless of appearance 4
• Euthermic/hypothermic high-risk patients: Presence of alternative infection indicators mandates aggressive evaluation even without fever 1, 2

Common Pitfalls to Avoid

• Inconsistent measurement sites: Always document measurement site with every reading to ensure comparability 2
• Relying on clinical appearance alone in infants: Inadequate sensitivity for serious bacterial infection 4
• Ignoring environmental factors in ICU: Specialized mattresses, hot lights, cardiopulmonary bypass, dialysis can alter measured temperature independent of infection 1, 2
• Fixed threshold in all children: Age-based thresholds improve diagnostic accuracy compared to fixed 38.0°C cutoff 6
• Dismissing infection in elderly with "normal" temperature: Lower baseline temperatures and functional decline may be only clues 2, 3