Definition and Terminology
Pyrexia and fever are the standard medical terms used interchangeably to describe an elevation in body temperature that occurs in response to infectious or noninfectious causes, associated with an increase in the hypothalamic set point 2
These terms are distinct from hyperthermia, which refers to temperature elevation without an increase in the hypothalamic set point (such as heat stroke or neuroleptic malignant syndrome) 2
Clinical Temperature Thresholds
The specific temperature that defines pyrexia varies by clinical context:
General adult ICU patients: A single temperature measurement ≥38.3°C (101°F) 3
Alternative definition: Core temperature ≥38.0°C (100.4°F) 3
Neutropenic cancer patients: A single oral temperature ≥38.3°C (101°F) OR a temperature ≥38.0°C (100.4°F) sustained over 1 hour 3
Pediatric patients: Rectal temperature ≥38.0°C (100.4°F) 3
Elderly in long-term care: Single oral temperature >37.8°C, repeated measurements >37.2°C (oral) or >37.5°C (rectal), or increase from baseline >1.1°C 3
Important Clinical Context
Normal body temperature is generally 37.0°C (98.6°F), with physiologic variation of 0.5-1.0°C based on circadian rhythm and menstrual cycle 3
The term "pyrexia of unknown origin" (PUO) specifically refers to fever ≥38.3°C lasting at least 3 weeks with no identified cause after appropriate investigation 1, 4
Not all infected patients manifest fever—absence of fever in infected patients is associated with worse outcomes 3