What is the definition of fever?

From the Guidelines

Fever is defined as a body temperature greater than or equal to 38.3°C, as per the Society of Critical Care Medicine and the Infectious Diseases Society of America guidelines for evaluating new fever in adult patients in the ICU 1. The definition of fever can vary depending on the context and population being studied. However, according to the most recent and highest quality study, a fever is typically defined as a single temperature measurement greater than or equal to 38.3°C 1. Some key points to consider when defining fever include:

• The normal body temperature range can vary depending on factors such as age, gender, and sampling site 1
• Different organizations, such as the Infectious Diseases Society of America, may have slightly different definitions of fever for specific populations, such as individuals greater than 65 years old or those with neutropenia due to chemotherapy 1
• The absence of fever in patients with infection can be associated with worse outcomes, highlighting the importance of considering other symptoms and clinical presentation when evaluating patients 1 It's worth noting that the definition of fever can impact treatment decisions and patient outcomes, and therefore it's essential to use a standardized and evidence-based definition, such as the one provided by the Society of Critical Care Medicine and the Infectious Diseases Society of America 1.

From the FDA Drug Label

PURPOSE Pain reliever/fever reducer Stop use and ask doctor if • pain gets worse or lasts more than 10 days • fever gets worse or lasts more than 3 days The FDA drug label does not answer the question.

From the Research

Definition of Fever

• Fever is defined as a regulated rise in body temperature that is defended by fully functional thermoregulatory mechanisms 2
• It is characterized by an upward displacement of the "set point" for body temperature regulation, which distinguishes it from other forms of hyperthermia 2
• Fever is often defined as a temperature greater than a certain value, but this definition ignores the importance of the anatomic site of temperature measurement and diurnal oscillations in body temperature 3

Key Characteristics of Fever

• Elevation in body temperature that is defended by fully functional thermoregulatory mechanisms 2
• Aspirin-like drugs can intervene in febrile hyperthermia and return body temperature to its normal level 2
• Whole-body cooling is ineffective in treating fever, whereas it is effective in treating other forms of hyperthermia 2

Clinical Definition of Fever

• Fever is defined as a clinically documented temperature of 101°F or higher on several occasions, coupled with an unrevealing diagnostic workup 4
• In children, fever is defined as a body temperature above the normal daily variation, which is 38°C (rectal or tympanic), 37.5°C (oral), or 37.2°C (axillary) 5

Biological Basis of Fever

• Fever is part of a systemic inflammatory response by the immune system to infection 5
• It involves the action of exogenous and endogenous pyrogens, which induce the production of pyrogenic cytokines and other molecules that mediate the fever response 3, 6