What is considered a fever?

What is Considered a Fever

Fever in adult ICU patients is defined as a single temperature measurement ≥38.3°C (101°F), while the CDC defines fever for hospital-acquired infections as >38°C (100.4°F), and these thresholds vary significantly based on patient population and clinical context. 1, 2

Standard Fever Definitions by Population

Adult ICU Patients

• A single core temperature ≥38.3°C (101°F) is the threshold recommended by the Society of Critical Care Medicine and Infectious Diseases Society of America for critically ill adults. 1, 2
• Alternative definitions exist: some sources use ≥38.0°C (100.4°F) as a single measurement, or two consecutive elevations of 38.3°C (101°F). 1

Pediatric Patients

• Fever in children younger than 3 years is defined as rectal temperature >38°C (100.4°F). 1, 2
• Rectal measurement is the preferred method in neonates and young children due to reliability concerns with other methods. 2

Neutropenic Patients

• A single oral temperature ≥38.3°C (101°F), OR temperature ≥38.0°C (100.4°F) sustained over 1 hour defines fever in neutropenic patients. 1, 2, 3
• This lower threshold reflects the critical nature of infections in immunocompromised patients. 3

Older Adults in Long-Term Care

• 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), OR increase from baseline >1.1°C constitutes fever in elderly residents. 1, 2
• This population-specific definition accounts for lower baseline temperatures in older adults. 2

Temperature Measurement Methods

Preferred Core Temperature Methods

• Pulmonary artery catheter thermistors, bladder catheter thermistors, and esophageal balloon thermistors are the gold standard for core temperature measurement when these devices are already in place. 1, 2
• Bladder thermistors provide readings essentially identical to intravascular sites with less invasiveness. 1

Practical Alternatives

• For patients without central monitoring devices, oral or rectal temperatures are recommended over less reliable methods such as axillary, tympanic, temporal artery, or chemical dot thermometers. 1, 2
• Rectal temperatures typically read a few tenths of a degree higher than core temperature. 1
• Oral temperature measurement is safe, convenient, and appropriate for alert, cooperative patients. 1

Critical Clinical Context

Normal Temperature Variability

• Normal body temperature is generally 37.0°C (98.6°F) but varies by 0.5-1.0°C due to circadian rhythm and menstrual cycle. 1
• Evidence shows human body temperature has decreased by 0.03°C per birth decade over the last 157 years. 1, 2
• Heavy exercise can raise temperature by 2-3°C without representing pathology. 1

Absence of Fever Does Not Exclude Infection

• A substantial proportion of infected patients are not febrile and may be euthermic or hypothermic, particularly the elderly, burn patients, those on extracorporeal support, patients with heart failure, end-stage liver disease, chronic renal failure, or those taking antipyretics. 1, 2
• Absence of fever in infected patients is associated with worse outcomes. 1, 2
• Other signs warranting infection workup include unexplained hypotension, tachycardia, tachypnea, confusion, rigors, oliguria, lactic acidosis, leukocytosis, leukopenia, or thrombocytopenia. 1

Common Pitfalls

Measurement Inconsistency

• Using different temperature measurement sites and methods yields significantly different readings, leading to diagnostic confusion. 2
• Axillary and tympanic measurements are unreliable and should be avoided when accurate assessment is critical. 1, 2

Age-Specific Threshold Failure

• Applying adult fever definitions to elderly patients or using general definitions for neutropenic patients leads to missed diagnoses. 2
• Older adults may have serious infections with temperatures that would not qualify as fever by standard adult definitions. 1, 2

Environmental Factors in ICU

• ICU-specific factors can alter temperature independent of infection: specialized mattresses, hot lights, air conditioning, cardiopulmonary bypass, peritoneal lavage, dialysis, and continuous hemofiltration. 1