What is Low-Grade Fever?
Low-grade fever is defined as a body temperature between 37.5°C and 38.3°C (99.5°F to 101°F), which falls below the threshold used for classic fever of unknown origin but still represents an elevation above normal body temperature. 1
Temperature Thresholds by Clinical Context
The definition of low-grade fever must be understood in relation to standard fever definitions across different populations:
- General adult ICU patients: Fever is defined as ≥38.3°C (101°F), making low-grade fever anything from 37.5°C to just below this threshold 2
- Elderly long-term care residents: A single oral temperature >37.8°C (100°F) already meets fever criteria, so low-grade fever in this population ranges from approximately 37.5°C to 37.8°C 2
- Normal body temperature baseline: The traditional 37.0°C (98.6°F) can vary by 0.5-1.0°C due to circadian rhythm and individual factors 2
Clinical Significance and Diagnostic Approach
Low-grade fever requires the same methodological diagnostic approach as classic fever of unknown origin, because there is no relationship between body temperature values and the severity of underlying diseases—the etiological spectrum is identical. 1
Common Causes of Organic Low-Grade Fever
When low-grade fever has an identifiable organic cause, the distribution is: 1
- Infectious diseases: 59% of cases
- Inflammatory non-infectious diseases: 6.2%
- Neoplasms: 3.1%
- Miscellaneous causes: 18.7%
- Undiagnosed: 12.5%
Distinguishing Organic from Non-Organic Forms
Physical examination findings and basic laboratory tests effectively differentiate organic fever from habitual hyperthermia (non-organic temperature elevation). 1
Features suggesting organic disease:
- Splenomegaly (p<0.05) 1
- Weight loss (p<0.05) 1
- Elevated white blood cell count (p<0.05) 1
- Elevated C-reactive protein (p<0.05) 1
- Any abnormal findings on physical examination 1
Features suggesting habitual hyperthermia (non-organic):
- Dizziness and general malaise without objective findings 1
- Complete absence of pathological signs on physical examination (p<0.0001) 1
- Normal inflammatory markers 1
- Younger age (mean age significantly lower than organic fever group, p<0.02) 1
Measurement Considerations
Always document the measurement site and method, as the term "fever" becomes meaningless without specifying the minimum temperature threshold, thermometer type, and measurement location. 3
- Oral measurements: Practical for cooperative patients but can be affected by recent fluid intake 2
- Rectal measurements: Read approximately 0.2-0.3°C higher than oral 2
- Central measurements (bladder, esophageal, pulmonary artery): Most accurate when available 2
Prognosis and Management Strategy
In approximately two-thirds of persistent low-grade fever cases, a definite diagnosis cannot be made despite extensive workup. 1, 4
Among undiagnosed cases: 4
- 14 of 19 cases (74%) had fever subside permanently within one year without treatment
- 5 of 19 cases (26%) had persistent fever but no deterioration in clinical status
- Sophisticated and expensive testing rarely changed outcomes in undiagnosed cases
Therefore, persistent low-grade fever should be managed more conservatively than classic fever of unknown origin to preserve healthcare resources, while maintaining vigilance for the specific diagnoses that do occur: dental granulomas, mycobacterial infections, thyroiditis, factitious fever, rheumatic polymyalgia, Hodgkin's lymphoma, and pulmonary thromboembolism. 4
Critical Pitfall: Low-Normal Temperatures in Severe Infection
Do not dismiss infection based on low-grade or even low-normal temperatures—approximately 22.5% of patients with severe community-acquired infection requiring ICU admission present with body temperatures ≤36.4°C, and hypothermia in infected patients is associated with worse outcomes. 5, 2