Normal Body Temperature in Adults
Normal body temperature is generally considered to be 37.0°C (98.6°F), though this varies by 0.5 to 1.0°C in healthy individuals according to circadian rhythm and other physiological factors. 1
Standard Temperature Range
The traditional reference point of 37.0°C (98.6°F) represents a historical benchmark rather than a universal constant. 1 However, recent evidence indicates that normal body temperature has been decreasing in the human population by 0.03°C per birth decade over the last 157 years, suggesting contemporary normal temperatures may be lower than this classic value. 1
Key Temperature Thresholds
- Normal (non-febrile): <38.0°C (100.4°F) according to the CDC and American College of Critical Care Medicine 2
- Fever threshold: ≥38.3°C (101°F) per Society of Critical Care Medicine and Infectious Diseases Society of America guidelines 1, 2
- Typical range in healthy adults: Mean temperatures can range from 35.2°C (95.4°F) to 37.4°C (99.3°F) between individuals 3
Physiological Variations
Body temperature exhibits predictable fluctuations that must be considered when interpreting measurements:
- Circadian rhythm: Temperature varies by 0.5 to 1.0°C throughout the day, with a nadir at 6 AM and peak at 4-6 PM 1
- Gender differences: Women have slightly higher normal temperatures than men 1
- Age effects: Older adults (≥60 years) have temperatures approximately 0.23°C lower than younger adults 4
- Exercise: Heavy physical activity can raise temperature by 2 to 3°C 1
- Menstrual cycle: Temperature varies according to hormonal fluctuations in women 1
Measurement Site Considerations
The accuracy of temperature measurement depends critically on the anatomical site used:
Most Accurate (Core Temperature)
- Pulmonary artery catheter thermistor: Gold standard for core temperature 1
- Bladder catheter thermistor: Provides readings essentially identical to intravascular sites 1
- Esophageal probe (distal third): Comparable to intravascular measurements 1
Clinically Practical Options
- Rectal temperature: Often a few tenths of a degree higher than core temperature; more accurate than oral or axillary methods 1
- Oral temperature: Safe, convenient, and familiar for alert, cooperative patients 1
Less Reliable (Avoid in Critical Settings)
- Axillary, tympanic membrane, temporal artery, or chemical dot thermometers: Should not be used when accurate measurements are critical 1, 5
Clinical Pitfalls and Special Populations
A critical caveat: the absence of fever does not exclude serious infection. 1, 6 Several patient populations may fail to mount a febrile response despite life-threatening infection:
- Elderly patients (who may have lower baseline temperatures) 1, 6
- Patients with open abdominal wounds or large burns 1, 6
- Those receiving extracorporeal membrane oxygenation or continuous renal replacement therapy 1, 6
- Patients with congestive heart failure, end-stage liver disease, or chronic renal failure 1, 6
- Those taking anti-inflammatory or antipyretic medications 1, 6
Alternative Infection Indicators When Fever is Absent
In euthermic or hypothermic patients, look for these signs that may indicate serious infection: 1, 5
- 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)
- Thrombocytopenia
Practical Recommendations
For routine clinical practice, use 37.0°C (98.6°F) as the reference point for normal body temperature, but recognize substantial individual variation exists. 1 Research demonstrates that 77% of healthy adults have mean temperatures at least 0.55°C (1°F) lower than 37.0°C, with marked thermal stability within individuals over time. 3
When fever is defined for clinical decision-making, use population-specific thresholds: