What is considered a normal body temperature?

Normal Body Temperature

Normal body temperature for healthy adults is approximately 36.8°C (98.2°F), not the traditional 37.0°C (98.6°F), with an upper limit of normal at 37.7°C (99.9°F) overall and 37.2°C (98.9°F) in early morning. 1

Core Temperature Range

• The mean oral temperature in healthy adults aged 18-40 years is 36.8°C (98.2°F), based on rigorous electronic thermometry studies 1
• Individual baseline temperatures vary substantially, ranging from 35.2°C (95.4°F) to 37.4°C (99.3°F) across healthy adults 2
• 77% of healthy individuals have mean temperatures at least 0.55°C (1°F) lower than the traditional 37.0°C (98.6°F) standard 2
• Evidence indicates human body temperature has been decreasing by 0.03°C per birth decade over the last 157 years 3

Diurnal Variation

• Body temperature follows a predictable daily pattern with a nadir at 6 AM and peak between 4-6 PM, with mean amplitude of variability of 0.5°C (0.9°F) 1
• This circadian rhythm must be considered when interpreting any single temperature measurement 1

Demographic Variations

• Women have slightly higher normal temperatures than men (approximately 0.2°C difference), even when environmental factors are controlled 2, 1
• Age does not significantly affect mean temperature in adults, though younger adults show greater day-to-day variability 2
• Black individuals trend toward slightly higher temperatures than white individuals 1

Measurement Site Considerations

• Central temperature monitoring (pulmonary artery catheter thermistors, bladder catheters, esophageal probes) represents the gold standard for core body temperature 3
• For routine clinical use, oral or rectal temperatures are preferred over less reliable methods (axillary, tympanic membrane, temporal artery, or chemical dot thermometers) 3
• Electronic digital thermometry is more accurate than mercury thermometry 4

Special Population Considerations

Elderly and Long-Term Care Residents

• Frail elderly individuals are at risk for lower baseline body temperatures 5
• A single oral temperature ≥37.8°C (100°F) has 70% sensitivity and 90% specificity for predicting infection in long-term care facility residents 3, 4
• Repeated oral temperatures ≥37.2°C (99°F) or rectal temperatures ≥37.5°C (99.5°F) meet fever criteria in this population 3

Critically Ill Patients

• The Society of Critical Care Medicine and Infectious Diseases Society of America define fever in ICU patients as a single temperature ≥38.3°C (101°F) 3

Clinical Implications

• Using 37.0°C (98.6°F) as a universal "normal" temperature leads to clinical errors, including failure to detect fever in individuals with low baseline temperatures 2
• Temperature shows marked stability within individuals over days, making personalized baseline measurements valuable 2
• Individual temperature baselines should ideally be established during routine health visits, similar to blood pressure monitoring 2

Critical Pitfall

The most common error is applying the outdated 37.0°C (98.6°F) standard universally. This 19th-century convention fails to account for substantial inter-individual variation and secular trends toward lower temperatures 6, 2, 1. When screening for infection or fever, clinicians must consider both the individual's baseline temperature and the specific population context (elderly, critically ill, immunocompromised) rather than relying on a single universal threshold 3, 4.