Temperature Range Definitions
Based on the most authoritative guideline evidence, normal body temperature is <38.0°C (100.4°F), low-grade fever is 38.0-38.3°C (100.4-100.9°F), and high-grade fever is ≥38.3°C (≥101°F). 1
Normal Body Temperature (Normothermia)
The traditional 37.0°C (98.6°F) standard is outdated and does not apply to most individuals. 2 Recent evidence shows normal body temperature has been decreasing by 0.03°C per birth decade over the last 157 years. 1
Normal oral temperature ranges from 36.1°C to 37.2°C (97.0-99.0°F) in healthy adults. 3 However, individual variation is substantial, with mean temperatures in one study ranging from 35.2°C to 37.4°C (95.4-99.3°F). 2
For clinical purposes, normothermia should be considered <38.0°C (100.4°F). 1 This threshold is used by the CDC for hospital-acquired infection surveillance and represents the upper limit before fever classification begins.
Special populations require adjusted thresholds: In elderly patients (>65 years) in long-term care, normal is defined as oral temperature <37.8°C (100.0°F) or rectal <37.5°C (99.5°F). 1
Low-Grade Fever
Low-grade fever is defined as body temperature between 37.5°C and 38.3°C (99.5-100.9°F). 4 This represents an elevation above normal but below the threshold for high-grade fever.
The CDC and IDSA define fever as >38.0°C (100.4°F), making 38.0-38.3°C the low-grade fever range. 1 This distinction is clinically relevant because low-grade fever requires the same diagnostic approach as fever of unknown origin, as disease severity does not correlate with temperature elevation. 4
High-Grade Fever
High-grade fever is defined as ≥38.3°C (≥101°F) based on SCCM/IDSA consensus. 1 This is the threshold used in ICU patients and in neutropenic patients receiving chemotherapy, where it is defined as either a single measurement ≥38.3°C or sustained temperature >38.0°C for at least 1 hour. 1
In high-risk populations, even lower thresholds carry significant mortality risk: Patients with neurologic injury (stroke, intracerebral hemorrhage) show increased mortality with fever >37.5°C (99.5°F), where even 1°F elevation above 99.6°F independently predicts poor outcomes. 5
Temperatures >40°C (104°F) require urgent investigation and treatment of the underlying cause. 5
Critical Clinical Considerations
Controlled normothermia (36.0-37.5°C) should be actively maintained in traumatic brain injury patients as part of Tier 1 and Tier 2 intracranial pressure management, with neurogenic fever >37.5°C treated promptly regardless of ICP level. 1
Absence of fever in infected patients paradoxically indicates worse prognosis, so these recommendations apply to suspected infection regardless of temperature elevation. 1
Temperature measurement site matters: Central methods (pulmonary artery, bladder, esophageal) are preferred when accurate measurements are critical; oral or rectal are acceptable alternatives, while axillary, tympanic, and temporal artery methods are unreliable. 1
Individual baseline temperature varies substantially (up to 2.2°C difference between individuals), so using a single "normal" value leads to diagnostic errors—77% of individuals have mean temperatures at least 0.55°C (1°F) lower than 37.0°C. 2