What are the grades of fever based on body temperature in adults?

Fever Grading in Adults

There is no universally standardized "grading system" for fever severity in general adult populations based solely on temperature thresholds. Instead, fever definitions vary by clinical context and patient population, with specific temperature cutoffs established for different settings rather than a tiered grading scheme 1, 2.

Standard Fever Definitions by Clinical Context

General Adult ICU Patients

• Fever is defined as a single temperature measurement ≥38.3°C (101°F) according to the Society of Critical Care Medicine (SCCM) and Infectious Diseases Society of America (IDSA) 1, 2
• This represents the most widely accepted threshold for critically ill adults 1

Hospital-Acquired Infections

• The CDC defines fever as >38.0°C (100.4°F) for diagnosing hospital-acquired infections 1
• This lower threshold increases sensitivity for detecting nosocomial infections 1

Elderly Patients in Long-Term Care

• Single oral temperature >37.8°C (100°F) is considered fever 1, 2
• Repeated measurements >37.2°C (oral) or >37.5°C (rectal) also qualify 1
• Temperature increase from baseline >1.1°C should prompt evaluation 1
• These lower thresholds account for blunted fever responses in elderly patients 1, 3

Neutropenic/Immunocompromised Patients

• Single oral temperature ≥38.3°C (101°F) defines fever 1, 2
• Temperature ≥38.0°C (100.4°F) sustained for ≥1 hour also qualifies 1, 2
• This population requires immediate evaluation due to high infection risk 2

Alternative Classification: Low-Grade vs High-Grade Fever

While not a formal grading system, clinical literature distinguishes:

Low-Grade Fever

• Temperature between 37.5°C and 38.3°C 4
• Requires the same diagnostic approach as higher fevers, as severity of underlying disease does not correlate with temperature magnitude 4

Moderate-to-High Fever

• Temperature ≥38.3°C 1, 2
• Associated with increased mortality risk when >39.5-39.7°C in hospitalized patients 5
• Temperatures >39.9°C independently predict increased mortality and acute kidney injury 5

Critical Temperature Thresholds for Adverse Outcomes

Fever magnitude correlates with clinical outcomes in specific populations:

• Temperature >39.7°C progressively increases mortality risk (OR 1.64-2.22) compared to 38.0-38.1°C 5
• Temperature >39.5°C increases acute kidney injury risk (OR 1.48-2.91) 5
• Temperature >39.7°C increases ICU admission risk 5
• Interestingly, temperatures between 39.2-39.5°C show paradoxically lower mortality (OR 0.62-0.71), suggesting a protective immune response window 5

Specialized Grading: Cytokine Release Syndrome

The only true "grading system" for fever exists in CAR T-cell therapy complications:

• Grade 1: Fever ≥38°C only, no hypotension or hypoxia 2
• Grade 2: Fever with hypotension not requiring vasopressors AND/OR hypoxia requiring low-flow oxygen 2
• Grades 3-4: Determined by severity of hypotension and hypoxia, not temperature 2
• After antipyretic therapy, fever is no longer required for grading—organ dysfunction alone determines severity 2

Critical Clinical Pitfalls

Absence of fever does not exclude serious infection 1, 2, 6:

High-Risk Populations for Absent/Blunted Fever Response

• Elderly patients 1, 6, 3
• Patients with open abdominal wounds or large burns 1, 2, 6
• Those receiving extracorporeal membrane oxygenation or continuous renal replacement therapy 1, 6
• Patients with end-stage liver disease, chronic renal failure, or congestive heart failure 1, 6
• Those taking anti-inflammatory or antipyretic medications 1, 6

Alternative Infection Indicators When Fever is Absent

• Unexplained hypotension, tachycardia, or tachypnea 1, 2, 6
• New confusion or altered mental status 1, 2, 6
• Rigors or new skin lesions 1, 2
• Oliguria or lactic acidosis 1, 2
• Leukocytosis, leukopenia, or ≥10% immature neutrophils (bands) 1, 2
• Elevated procalcitonin or C-reactive protein 6

The absence of fever in infected patients paradoxically predicts worse outcomes 1, 6, making vigilance for alternative signs essential 1, 2.

Temperature Measurement Hierarchy

Accuracy varies significantly by measurement method 1, 2:

Most Accurate (Gold Standard)

• Pulmonary artery catheter thermistor 1, 2
• Bladder catheter thermistor 1, 2
• Esophageal thermistor 1, 2

Acceptable Alternatives

• Rectal thermometer (few tenths of a degree variation from core) 1
• Oral thermometer (safe and convenient for alert patients, but affected by mouth breathing and recent hot/cold intake) 1

Unreliable Methods (Avoid in Critical Care)

• Axillary measurements 1, 2
• Tympanic membrane thermometers (often 1-2 degrees different from core) 1
• Temporal artery thermometers 1, 2
• Chemical dot thermometers 1, 2

Practical Clinical Approach

When evaluating fever severity, consider:

1. Patient population determines which temperature threshold applies 1, 2
2. Absolute temperature >39.5°C warrants aggressive evaluation regardless of population 5
3. Trend from baseline may be more important than absolute value in elderly patients 1
4. Clinical context and associated symptoms outweigh temperature magnitude for determining infection severity 1, 2, 4
5. Normal body temperature has been decreasing over time (0.03°C per birth decade over 157 years), so historical norms may not apply 1, 6