Fever Definition and Assessment Guidelines

Fever Definitions by Patient Population and Setting

The American College of Critical Care Medicine defines fever in adult ICU patients as a single temperature measurement greater than or equal to 38.3°C (101°F) 1, 2, 3
The Centers for Disease Control and Prevention (CDC) defines fever for hospital-acquired infections as temperature >38°C (100.4°F) 1, 2
The Infectious Diseases Society of America recommends defining fever in older adults in long-term care facilities as a single oral temperature >37.8°C (100°F), or repeated oral temperatures >37.2°C (99°F) or rectal temperatures >37.5°C (99.5°F) 4, 5
The American Society of Clinical Oncology defines fever in neutropenic patients as a single oral temperature >38.3°C (101°F), or temperature ≥38.0°C (100.4°F) sustained over a 1-hour period 6, 7
The American Academy of Pediatrics defines fever in pediatric patients as temperature ≥38°C (100.4°F) 8, 9

The Society of Critical Care Medicine recommends using central temperature monitoring methods, such as pulmonary artery catheter thermistors, bladder catheter thermistors, and esophageal balloon thermistors, when available 1, 2, 3
For patients without central monitoring devices, the American College of Critical Care Medicine recommends using oral or rectal temperatures over less reliable methods 1, 2, 3
The American Academy of Pediatrics recommends using rectal temperature in neonates and young children, and oral temperature in older, cooperative children 8, 9

Normal body temperature range is influenced by age, gender, diurnal variation, and measurement site, according to the American College of Critical Care Medicine 1, 2
Evidence indicates that normal body temperature has been decreasing in humans by 0.03°C per birth decade over the last 157 years, as reported by the American College of Critical Care Medicine 1, 2
The Infectious Diseases Society of America notes that not all infections manifest with fever, and the absence of fever in infected patients is associated with worse outcomes 1, 2
The American Geriatrics Society recommends suspecting infection in older adults with new or increasing confusion, incontinence, falling, deteriorating mobility, reduced food intake, or failure to cooperate with staff, even in the absence of fever 4, 5

The Society of Critical Care Medicine warns against inconsistent temperature measurement methods, which can yield significantly different readings, and recommends using central thermometers and rectal measurements over oral or axillary methods 1, 2, 3
The American Geriatrics Society notes that failure to consider age-specific definitions of fever can lead to misdiagnosis, particularly in older adults who may have lower baseline temperatures and different fever thresholds 4, 5
The Infectious Diseases Society of America recommends considering non-temperature signs of infection, such as functional decline, particularly in older adults 4, 5