Fever Criteria and Assessment

Definition and Interpretation

A single oral temperature of ≥100°F (37.8°C) is considered fever with high specificity (90%) for infection, according to the Infectious Diseases Society of America 1, 2
Repeated oral temperatures ≥99°F (37.2°C) or rectal temperatures ≥99.5°F (37.5°C) meet criteria for fever, as recommended by the American Geriatrics Society 1, 3
An increase in temperature of ≥2°F (≥1.1°C) over the individual's baseline temperature is also considered fever, regardless of the absolute value, according to the Clinical Infectious Diseases guidelines 1, 4

A temperature of 99.3°F should be evaluated in context of the individual's known baseline temperature, whether it represents a sustained or repeated elevation, and presence of other symptoms or signs of infection, as suggested by the Clinical Infectious Diseases guidelines 1, 2, 4
Changes in functional status, especially in older adults (confusion, decreased mobility, etc.), should be considered when evaluating fever, according to the American College of Physicians 6, 1

In older adults, particularly those in long-term care facilities, fever thresholds are often lower, and an oral temperature of ≥99°F (37.2°C) on repeated measurements is considered fever, as recommended by the American Geriatrics Society 6, 1, 5
Changes in mental status or functional decline may be more reliable indicators of infection than fever in older adults, according to the Clinical Infectious Diseases guidelines 6, 3

Method of temperature measurement affects interpretation, with oral temperatures being most commonly used but may be less accurate than rectal measurements, as noted by the Infectious Diseases Society of America 1, 4
Electronic thermometry is generally more accurate than standard mercury thermometry, according to the Journal of the American Geriatrics Society 4, 5

Failing to consider repeated measurements when temperatures are in the borderline range (99-100°F) can lead to missed fevers, as warned by the Clinical Infectious Diseases guidelines 1, 2
Overlooking non-temperature indicators of infection, especially in older adults, can delay diagnosis, according to the American College of Physicians 6, 3
Not accounting for the method of temperature measurement (oral vs. rectal vs. tympanic) can affect accuracy, as noted by the Infectious Diseases Society of America 4, 5