From the Guidelines
Fever is defined as a measured temperature of greater than 38°C, according to the United States Centers for Disease Control and Prevention, although different definitions may apply in specific contexts, such as in patients with neutropenia or the elderly. The definition of fever can vary depending on the population being studied and the purpose of the definition. For example, in patients with neutropenia due to chemotherapy, fever is defined by both the IDSA and the National Comprehensive Cancer Network as a single oral temperature measurement greater than or equal to 38.3°C or greater than 38.0°C sustained over at least 1 hour 1. In patients greater than 65 years old residing in long-term care facilities, the Infectious Diseases Society of America (IDSA) has defined fever as a single oral temperature greater than 37.8°C, repeated temperature measurements greater than 37.2°C (oral) or greater than 37.5°C (rectal), or an increase from baseline greater than 1.1°C 1.
The normal body temperature range is subject to various factors such as age, gender, diurnal variation, and sampling site, and evidence suggests that the normal body temperature has been decreasing in the human population over the last 157 years 1. Not all patients with infection manifest fever, and the absence of fever in patients with infection is associated with worse outcomes 1. Therefore, it is essential to consider other symptoms and signs in the absence of fever when evaluating patients for possible infection.
Some key points to consider when defining fever include:
- The definition of fever can vary depending on the population and context
- Different organizations, such as the IDSA and the National Comprehensive Cancer Network, may have different definitions of fever
- Fever can be defined as a single temperature measurement or as a sustained temperature elevation over a period of time
- The absence of fever in patients with infection can be associated with worse outcomes, and other symptoms and signs should be considered when evaluating patients for possible infection 1.
In clinical practice, it is crucial to use a definition of fever that is appropriate for the specific patient population and context, and to consider other symptoms and signs when evaluating patients for possible infection.
From the Research
Definition of Fever
- Fever is a complex physiological response involving the innate immune system and should not be characterized merely as a temperature above the normal threshold of 37.0°C (98.6°F) 2
- It is defined by a temperature of >or=38.3 degrees C (101 degrees F) 3
- Fever is a regulated rise in body temperature that is defended by fully functional thermoregulatory mechanisms, with an upward displacement of the "set point" for body temperature regulation 4
Characteristics of Fever
- The elevation in body temperature encountered during fever is a regulated rise that is defended by fully functional thermoregulatory mechanisms 4
- The thermopreferendum is also elevated in fever, particularly at the onset or "chill phase" 4
- Aspirin-like drugs can intervene in febrile hyperthermia and will return body temperature to its normal level 4
Distinction from Hyperthermia
- Fever differs markedly from all other forms of hyperthermias, which do not possess the characteristics of a regulated rise in body temperature and an upward displacement of the "set point" for body temperature regulation 4
- Hyperthermia is a rise in body temperature that is not accompanied by supportive changes in thermoeffector activities 5