Temperature Ranges for Low-Grade and High-Grade Fever in Celsius

Low-grade fever is defined as a body temperature between 37.5°C and 38.3°C, while high-grade fever is considered a temperature above 38.3°C. 1, 2

Fever Definitions and Classifications

A single oral temperature of ≥37.8°C (100°F) is considered fever with high specificity (90%) for infection 2
Repeated oral temperatures ≥37.2°C (99°F) or rectal temperatures ≥37.5°C (99.5°F) also meet criteria for fever 2
An increase in temperature of ≥1.1°C (≥2°F) over an individual's baseline temperature is considered fever, regardless of the absolute value 2
High-grade fever is defined as temperature ≥38.3°C (101°F) according to the American College of Critical Care Medicine 3

Low-grade fever (37.5-38.3°C) requires the same methodological diagnostic approach as fever of unknown origin, as there is no relationship between temperature values and severity of underlying diseases 1
Temperatures above 39.5°C are associated with adverse clinical outcomes compared to lower-grade fever (38.0-38.1°C) 4
Temperatures above 39.7°C are progressively associated with increased mortality risk (OR 1.64-2.22) compared to temperatures of 38.0-38.1°C 4
Temperatures between 39.2°C and 39.5°C have been associated with lower mortality in some studies (OR 0.62-0.71) 4

Normal body temperature varies significantly between individuals, with mean temperatures ranging from 35.2°C to 37.4°C 5
Women typically have higher baseline temperatures than men 5
In older adults, particularly those in long-term care facilities, fever thresholds are often lower, with an oral temperature of ≥37.2°C (99°F) on repeated measurements considered fever 2, 6
The Centers for Disease Control and Prevention (CDC) defines fever for hospital-acquired infections as temperature >38°C (100.4°F) 3

Method of temperature measurement affects interpretation:
- Oral temperatures are most commonly used but may be less accurate than rectal measurements 2
- Electronic thermometry is generally more accurate than standard mercury thermometry 6
- Central temperature monitoring methods (pulmonary artery catheters, bladder catheters) are most reliable in critically ill patients 3

Using a single standardized "normal" temperature (37.0°C/98.6°F) for all individuals can lead to missed fevers in those with naturally lower baseline temperatures 5
Failing to consider repeated measurements when temperatures are in the borderline range (37.0-37.8°C) can lead to missed diagnoses 2
Not accounting for the method of temperature measurement (oral vs. rectal vs. tympanic) can affect accuracy of fever assessment 2
Overlooking non-temperature indicators of infection, especially in older adults, can delay diagnosis 2