Fever Definition by Temperature Measurement Site in Children
Fever in children is defined as a rectal temperature ≥38.0°C (100.4°F), which is the gold standard measurement. 1
Temperature Thresholds by Measurement Site
The American College of Emergency Physicians clinical policy establishes rectal temperature ≥38.0°C (100.4°F) as the standard definition of fever in children younger than 2 years 1. However, different measurement sites require adjusted thresholds:
Site-Specific Fever Definitions:
- Rectal: ≥38.0°C (100.4°F) - Gold standard 1, 2, 3
- Oral: ≥37.5°C 2
- Tympanic (ear): ≥37.6-37.8°C 2, 3, 4
- Axillary: ≥37.0-37.5°C 2, 3, 4
Age-Specific Considerations
For infants 0-2 months old, fever thresholds are slightly higher:
- Axillary: ≥37.5°C (99th percentile)
- Tympanic: ≥37.85°C (99th percentile) 4
This reflects the physiologically higher baseline temperatures in very young infants.
Accuracy and Clinical Reliability
Rectal Measurement
Remains the most accurate and reliable method for detecting fever in children, particularly those under 2 years where missing fever can have serious consequences 1, 3, 5, 6.
Tympanic (Ear) Measurement
- Shows strong correlation with rectal temperature (r=0.91) 5
- Sensitivity of 71-76% for detecting fever 3, 7
- Better accuracy than axillary measurement 3, 5
- Most reliable in children aged 6 months to 5 years 6
- Mean difference from rectal: 0.38-0.42°C 3
Axillary Measurement
- Significantly less accurate than tympanic or rectal 3, 5, 6
- Sensitivity only 24-54% for detecting fever 3, 5
- Mean difference from rectal: 1.11-1.58°C 3
- Not recommended as primary screening tool due to poor sensitivity 6
Temporal (Forehead) Measurement
- Not currently recommendable for clinical use 6
- 95% limits of agreement: -1.2 to +1.5°C from rectal 6
- Lower sensitivity than tympanic measurement 6
Clinical Algorithm for Temperature Measurement
For children under 2 years (especially <3 months):
- Use rectal measurement as first-line when feasible
- If rectal not possible, use tympanic measurement for screening
- Never rely on axillary measurement alone to rule out fever
For children 6 months to 5 years:
- Tympanic measurement acceptable for screening (sensitivity >90%) 6
- Confirm with rectal if clinical suspicion remains high despite normal tympanic reading
Critical caveat: The guideline emphasizes that children may have serious bacterial infections despite being hypothermic or having normal temperature, particularly if antipyretics were given within 4 hours of presentation 1. Clinical appearance and risk factors must always supersede temperature readings alone.
Key Pitfalls to Avoid
- Do not use home thermometer readings without verification of device accuracy 1
- Do not exclude serious infection based solely on axillary temperature - sensitivity is too low 3, 5
- Do not assume viral infection precludes bacterial co-infection 1
- Do not overlook recent antipyretic use which may mask fever 1