Definition of Fever Based on Temperature Measurement Site
Fever is defined as ≥38.3°C (≥100.9°F) for oral measurements, ≥37.5°C (≥99.5°F) for axillary measurements, and ≥38.0°C (≥100.4°F) for tympanic measurements, with central temperature monitoring methods being the gold standard when available. 1, 2
Temperature Measurement Site-Specific Fever Definitions
Oral Temperature
- A single oral temperature ≥38.3°C (≥100.9°F) is considered fever according to Society of Critical Care Medicine and Infectious Diseases Society of America guidelines 1
- For elderly patients in long-term care facilities, fever is defined as a single oral temperature >37.8°C (>100°F) or repeated oral temperatures >37.2°C (>99°F) 1
- In patients with neutropenia, fever is defined as a single oral temperature ≥38.3°C (≥101°F) or ≥38.0°C (≥100.4°F) sustained over at least 1 hour 1, 2
Axillary Temperature
- Axillary temperature ≥37.5°C (≥99.5°F) is considered fever 1, 3
- In pediatric patients, axillary temperature >37.0°C is considered fever, with a higher threshold of >37.5°C for infants 0-2 months 4
- Axillary measurements are generally less reliable than oral or rectal measurements and often underestimate core body temperature 1
Tympanic Temperature
- Tympanic temperature ≥37.6-37.8°C (≥99.7-100°F) is generally considered fever 2, 4
- In pediatric patients, tympanic temperature >37.8°C is considered fever, with a higher threshold of >37.85°C for infants 0-2 months 4
- Tympanic measurements can be inaccurate if there is inflammation or blockage of the auditory canal or if the thermometer is not properly positioned 1
Measurement Method Reliability
Gold Standard Methods
- Central temperature monitoring methods are considered the gold standard for measuring core body temperature 1, 2
- These include thermistors in pulmonary artery catheters, bladder catheters, or esophageal balloon thermistors 1
- When these devices are not in place, rectal temperature is often considered the next most reliable method 1, 5
Comparative Reliability
- Oral temperature measurements are more reliable than axillary or tympanic measurements but can be affected by mouth breathing or recent consumption of hot/cold food or beverages 1
- Axillary measurements are consistently less accurate and often underestimate core temperature by 0.5-1.5°C 1, 4, 3
- Tympanic measurements show variable reliability across studies, with some showing good correlation with rectal temperatures in pediatric patients 6 while others demonstrate poor sensitivity in adults 7
Clinical Considerations and Pitfalls
Age-Specific Considerations
- Normal body temperature varies by age, with elderly patients often having lower baseline temperatures 1, 2
- In elderly patients, an increase in temperature of ≥2°F (1.1°C) over baseline may indicate fever even if absolute temperature thresholds aren't met 1
- Infants and young children may have different normal temperature ranges and fever thresholds 4
Common Pitfalls
- Using inconsistent temperature measurement methods can lead to significant variations in readings 2
- Failing to consider the measurement site when interpreting temperature results 8
- Not accounting for factors that can affect temperature readings, such as environmental conditions, recent physical activity, or medications 5
- Relying solely on temperature without considering other signs of infection, particularly in elderly patients who may not mount typical fever responses 1, 5
Best Practices
- Use the same measurement site and method consistently when monitoring a patient's temperature 1, 2
- When accurate temperature measurement is critical for diagnosis and management, use central temperature monitoring methods if available 1
- In patients without central monitoring devices, oral or rectal temperatures are preferred over axillary or tympanic measurements 1
- Consider non-temperature signs of infection, particularly in populations that may not mount typical fever responses 1, 5