At what body temperature is a fever typically considered present in an adult patient?

Fever Definition in Adults

A fever is defined as a single temperature measurement ≥38.3°C (101°F) in adult ICU patients according to the most recent 2023 Society of Critical Care Medicine and Infectious Diseases Society of America guidelines. 1

Context-Specific Temperature Thresholds

The definition of fever varies by clinical setting and patient population:

General Adult Populations

• Standard threshold: The CDC defines fever as >38.0°C (100.4°F) for hospital-acquired infection surveillance 1
• ICU patients: A single measurement ≥38.3°C (101°F) is the established criterion 1, 2
• Alternative definition: Some sources accept two consecutive elevations of 38.3°C (101°F) 1

Special Populations Requiring Lower Thresholds

• Elderly patients (>65 years) in long-term care facilities: A single oral temperature ≥37.8°C (100°F) is both sensitive (70%) and specific (90%) for predicting infection 1, 3
• Alternative elderly criteria: Repeated oral temperatures ≥37.2°C (99°F), rectal temperatures ≥37.5°C (99.5°F), or an increase from baseline ≥1.1°C (2°F) 1

High-Risk Immunocompromised Patients

• Neutropenic patients: A single oral temperature ≥38.3°C (101°F) OR a sustained temperature ≥38.0°C (100.4°F) for at least 1 hour 1, 3

Temperature Measurement Hierarchy

Use the most accurate method available, as measurement site significantly affects readings:

• Gold standard: Central thermistors (pulmonary artery catheter, bladder catheter, esophageal probe) 1, 3
• Acceptable alternatives: Oral or rectal electronic thermometers when central monitoring unavailable 1, 4
• Avoid: Axillary, tympanic membrane, temporal artery, or chemical dot thermometers in critical care settings due to unreliability 1, 3

Critical Clinical Pitfall: Absence of Fever Does Not Exclude Infection

A substantial proportion of infected patients never develop fever, and these patients have worse outcomes. 1, 2, 4, 3

High-Risk Groups for Absent Fever Response

• Elderly patients 4, 5
• Patients with end-stage liver disease, chronic renal failure, or congestive heart failure 1, 4
• Those receiving anti-inflammatory drugs, antipyretics, or corticosteroids 1, 4
• Patients with open abdominal wounds or large burns 1, 4, 3
• Those receiving extracorporeal membrane oxygenation or continuous renal replacement therapy 1, 4, 3

Alternative Infection Indicators When Fever is Absent

Proceed with comprehensive infection workup if any of the following are present:

• Unexplained hypotension, tachycardia, or tachypnea 1, 4, 3
• New confusion or altered mental status 1, 4, 3
• Decline in functional status or inability to perform usual activities of daily living 1
• Rigors or new skin lesions 1, 4
• Oliguria or lactic acidosis 1, 4
• Leukocytosis, leukopenia, ≥10% immature neutrophils (bands), or thrombocytopenia 1, 4, 3

Practical Temperature Considerations

• Normal body temperature has been decreasing by 0.03°C per birth decade over the last 157 years 1, 4
• Basal temperatures in frail elderly may be lower than the traditional 37.0°C (98.6°F) 1, 5
• Normal temperature varies by 0.5-1.0°C due to circadian rhythm and menstrual cycle 1
• ICU environmental factors (specialized mattresses, hot lights, cardiopulmonary bypass, dialysis) can alter measured temperature 1