What are the different types of fever and their characteristics?

Types of Fever and Their Characteristics

Fever is broadly categorized by its temporal pattern (continuous, remittent, intermittent, hectic, or sustained) and by underlying etiology (infectious, inflammatory, neoplastic, drug-induced, or miscellaneous causes), though fever patterns have limited diagnostic utility in distinguishing specific diseases. 1, 2

Fever Definitions by Clinical Context

The definition of fever varies substantially based on patient population and clinical setting:

• General adult population: Temperature >38.0°C (100.4°F) for hospital-acquired infections 3
• ICU patients: Single temperature ≥38.3°C (101.0°F) 3
• Elderly in long-term care: Single oral temperature >37.8°C, repeated measurements >37.2°C (oral) or >37.5°C (rectal), or increase from baseline >1.1°C 3
• Neutropenic patients: Single oral temperature ≥38.3°C or ≥38.0°C sustained over 1 hour 3
• Infants: Rectal temperature ≥38.0°C (100.4°F) 4

Normal body temperature has been decreasing by 0.03°C per birth decade over the last 157 years, making historical thresholds less applicable. 3

Fever Patterns and Their Limited Clinical Significance

Temporal Patterns

Despite traditional teaching, fever patterns rarely provide diagnostic specificity and should not guide clinical decision-making. 2

• Continuous fever: Persistent elevation with <1°C variation over 24 hours 1
• Remittent fever: Daily fluctuations >1°C but never reaching normal, typically following diurnal variation 1, 2
• Intermittent fever: Temperature returns to normal between spikes 1, 2
• Hectic fever: Dramatic swings with temperature spikes and returns to normal, historically associated with bacteremia but occurs in all disease categories 2
• Sustained fever: Persistent elevation without significant variation, most commonly seen in gram-negative pneumonia or CNS damage 2

A prospective study of 200 consecutive patients found that hectic fever occurred in patients with all categories of infectious and noninfectious diseases, and while more frequent in bacteremia, many nonbacteremic patients had this pattern while others with bacteremia did not. 2

Etiologic Classification

Infectious Causes

Infectious etiologies remain the most common cause of fever, accounting for the majority of cases in most clinical settings. 1, 5, 6

• Bacterial infections: Including bacteremia, pneumonia, urinary tract infections, endocarditis 3, 7
• Viral infections: Common in returning travelers (dengue 78% present with fever, malaria 92%) 3
• Atypical infections: Q fever (Coxiella burnetii), tularemia (with distinct forms: ulceroglandular, glandular, oculoglandular, oropharyngeal, intestinal, pneumonic, typhoidal) 3, 7
• Parasitic infections: Katayama syndrome (acute schistosomiasis) presenting 4-6 weeks after freshwater exposure 7

Inflammatory/Rheumatologic Causes

• Rheumatic fever: Delayed sequela of group A streptococcal infection with major criteria (carditis, polyarthritis, chorea, subcutaneous nodules, erythema marginatum) and minor criteria (previous rheumatic fever, arthralgia, fever, elevated inflammatory markers, prolonged PR interval) 3
• Kawasaki disease: Fever ≥5 days plus ≥4 of 5 principal features (bilateral conjunctival injection, oral mucosal changes, polymorphous rash, extremity changes, cervical lymphadenopathy ≥1.5 cm), with incomplete forms common in infants <1 year 7
• Multisystem Inflammatory Syndrome in Children (MIS-C): Temporally associated with SARS-CoV-2 infection 2-6 weeks post-exposure, with significantly higher temperatures and longer fever duration 7

Neoplastic Causes

Malignancies account for a significant portion of fever of unknown origin cases, particularly leukemia and lymphoma. 7, 6

• Lymphoma often presents with markedly elevated lactate dehydrogenase 7
• Acute lymphoblastic leukemia may present with prolonged fever, lymphadenopathy, and cytopenias 7

Drug-Induced Fever

Drug fever occurs with a mean lag time of 21 days (median 8 days) after drug initiation and can take 1-7 days to resolve after discontinuation. 7

• Can be caused by any medication through hypersensitivity mechanisms 7
• Rash and eosinophilia are uncommon, making diagnosis challenging 7
• Neuroleptic malignant syndrome: From antipsychotics (especially haloperidol in ICU settings) with muscle rigidity and elevated creatinine phosphokinase 7
• Malignant hyperthermia: From anesthetics with delayed onset up to 24 hours 7

Miscellaneous Causes

• Venous thromboembolism 6
• Thyroiditis 6
• Genetic abnormalities of inflammasome functioning 1
• Immune or metabolic disorders 1

Fever of Unknown Origin (FUO)

FUO is defined as clinically documented temperature ≥101°F (38.3°C) on several occasions coupled with an unrevealing diagnostic workup. 6, 8

• Most cases (up to 75%) resolve spontaneously without reaching a definitive diagnosis 6
• Approximately 50% of pediatric FUO cases will have self-limited illness and never receive a specific diagnosis 7
• Most adult cases occur because of uncommon presentations of common diseases 6

Critical Clinical Pitfalls

Temperature Measurement

Central temperature monitoring (pulmonary artery catheter thermistors, bladder catheters, esophageal balloon thermistors) is the gold standard; oral or rectal temperatures are preferred over axillary, tympanic, temporal artery, or chemical dot thermometers when central monitoring is unavailable. 3

• Oral temperatures suffer from poor sensitivity to diagnose fever 5
• Never rely on parental report of "feeling warm" or alternative home measurements 4

Absence of Fever Does Not Exclude Infection

A substantial proportion of infected patients are not febrile, and absence of fever in patients with infection is associated with worse outcomes. 3

Patients at risk for euthermic or hypothermic infection include:

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

Biomarker Limitations

Adjunctive testing including C-reactive protein, erythrocyte sedimentation rate, and procalcitonin lacks the sensitivity and specificity to definitively rule in bacterial causes of fever. 5

• If probability of bacterial infection is low to intermediate in ICU patients, measure procalcitonin or CRP in addition to clinical evaluation 3
• If probability is high, do not measure PCT or CRP to rule out bacterial infection 3

Empiric Therapy Considerations

Empiric antimicrobial therapy has not been shown to be effective in treatment of FUO and should be avoided except in neutropenic, immunocompromised, or critically ill patients. 6