What is considered a low-grade fever?

Definition of Low-Grade Fever

A low-grade fever is generally defined as a body temperature between 37.5°C (99.5°F) and 38.3°C (100.9°F), which is elevated above normal but below the threshold for a clinically significant fever. 1

Temperature Measurement Methods and Normal Values

Temperature measurement varies by method and site:

• Core temperature: Best measured using central methods like pulmonary artery catheter thermistors, bladder catheters, or esophageal thermistors 2
• Oral temperature: Normal is approximately 37.0°C (98.6°F), with diurnal variations of 0.5-1.0°C 2
• Rectal temperature: Typically 0.3-0.6°C higher than oral temperature 2
• Axillary temperature: Usually 0.5-1.0°C lower than oral and 1.15°C lower than rectal temperature 3

Clinical Significance of Low-Grade Fever

Low-grade fever requires careful evaluation as it may indicate underlying pathology:

• In a study of patients with low-grade fever, 59% had infectious causes, 3.1% had neoplasms, 6.2% had inflammatory non-infectious diseases, and 18.7% had miscellaneous causes 1
• 12.5% remained undiagnosed despite thorough evaluation 1

Distinguishing Organic from Non-Organic Low-Grade Fever

Clinical Features Associated with Organic Causes:

• Splenomegaly
• Weight loss
• Elevated white blood cell count
• Elevated C-reactive protein 1

Features Associated with Habitual Hyperthermia (Non-Organic):

• Dizziness
• General malaise
• Normal physical examination
• Normal laboratory values 1

Special Populations and Considerations

Elderly Patients

• An increase of 1.1°C above baseline may be significant 3
• The Infectious Diseases Society of America defines fever in individuals >65 years in long-term care as:
  • Single oral temperature >37.8°C
  • Repeated oral temperatures >37.2°C
  • Repeated rectal temperatures >37.5°C 2

Critically Ill Patients

• The Society of Critical Care Medicine defines fever as ≥38.3°C (101°F) 2
• Absence of fever in infected patients may indicate worse outcomes 2, 3

Neutropenic Patients

• Fever defined as single oral temperature ≥38.3°C or ≥38.0°C sustained over at least 1 hour 2

Common Misconceptions

Many parents and some healthcare providers exhibit "fever phobia," with undue concern about low-grade fevers 4. Research shows that:

• The only serious direct complications of fever are febrile status epilepticus and heat stroke, both rare entities 4
• Moderate fever (≤40°C) does not cause neurological damage, contrary to common belief 4

Clinical Approach

When evaluating low-grade fever:

1. Confirm accurate temperature measurement using appropriate method
2. Consider patient's baseline temperature and diurnal variations
3. Evaluate for associated symptoms and physical examination findings
4. Order appropriate laboratory tests when indicated (CBC, CRP)
5. Apply the same methodological diagnostic approach as for fever of unknown origin when persistent 1

Low-grade fever requires thorough evaluation as there is no relationship between temperature values and the severity of underlying diseases 1.