What is the definition of a fever in Celsius?

Fever Definition in Celsius

Fever is defined as a core body temperature ≥38.3°C (101°F) in adult ICU patients according to the Society of Critical Care Medicine and Infectious Diseases Society of America. 1

Standard Definitions by Clinical Context

The threshold for fever varies based on patient population and clinical setting:

Adult Populations

• ICU patients: Single temperature measurement ≥38.3°C is the standard definition 1, 2
• Hospital-acquired infections: ≥38.0°C per CDC criteria 1, 2
• Long-term care facilities (elderly >65 years): Single oral temperature ≥37.8°C, or repeated measurements ≥37.2°C (oral) or ≥37.5°C (rectal), or increase from baseline >1.1°C 1
• General critical care: Some sources accept ≥38.0°C as fever, though ≥38.3°C provides greater specificity 1

Pediatric Populations

• Children <3 years: Rectal temperature ≥38.0°C (100.4°F) per American Academy of Pediatrics 1, 2

Immunocompromised Patients

• Neutropenic patients: Single oral temperature ≥38.3°C OR sustained temperature ≥38.0°C for ≥1 hour 1, 2

Measurement Site Hierarchy

Central temperature monitoring provides the most accurate readings and should be prioritized when available. 1

The accuracy hierarchy from most to least reliable is:

• Intravascular thermistor (pulmonary artery catheter) - gold standard 1, 2
• Bladder thermistor - essentially identical to intravascular, less invasive 1
• Esophageal thermistor - comparable accuracy but placement challenges 1, 2
• Rectal temperature - traditional method, often 0.1-0.3°C higher than core 1
• Oral temperature - safe and convenient for alert patients 1

Avoid axillary, temporal artery, or chemical dot thermometers in critical care settings as they lack reliability. 2, 3

Critical Clinical Pitfalls

Absence of Fever Does Not Exclude Serious Infection

Many infected patients never develop fever, and this absence paradoxically predicts worse outcomes. 1, 2

High-risk populations for absent or blunted fever response include:

• Elderly patients 1
• Patients with open abdominal wounds or large burns 1, 2
• Those receiving extracorporeal membrane oxygenation or continuous renal replacement therapy 1, 2
• Patients with end-stage liver disease, chronic renal failure, or congestive heart failure 1
• Those taking anti-inflammatory or antipyretic medications 1

Alternative Infection Indicators When Fever is Absent

Monitor for these signs in afebrile patients with suspected infection:

• Unexplained hypotension, tachycardia, or tachypnea 1, 2
• New confusion or altered mental status 1, 2
• Rigors or new skin lesions 1
• Oliguria or lactic acidosis 1, 2
• Leukocytosis, leukopenia, or ≥10% immature neutrophils (bands) 1, 2
• Thrombocytopenia 1

Context-Specific Variations

Normal Temperature Considerations

• Normal body temperature is traditionally 37.0°C 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 past 157 years 1, 3
• Heavy exercise can raise temperature by 2-3°C 1

Environmental Factors in ICU

Multiple ICU interventions can alter measured temperature independent of infection:

• Specialized mattresses, hot lights, air conditioning 1
• Cardiopulmonary bypass, peritoneal lavage 1
• Dialysis and continuous hemofiltration 1

Disease-Specific Thresholds

In acute rheumatic fever, fever definitions vary by population risk: 1

• High-risk populations: >38.0°C provides better sensitivity (75% vs 25% with >39.0°C cutoff) 1
• Low-risk populations: Fever typically exceeds 38.5°C orally 1