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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Classic FUO: temperature >38.3 C on multiple occasions, duration >3 weeks, and no diagnosis after appropriate evaluation
- Major categories: infection, malignancy, noninfectious inflammatory disease, miscellaneous, and undiagnosed
- Initial approach is careful history, repeated exam, medication/exposure review, basic labs, cultures, urinalysis, chest imaging, and targeted testing
- Do not give empiric antibiotics in stable FUO unless neutropenia, suspected TB meningitis, culture-negative endocarditis, or clinical instability
- Temporal artery biopsy is high-yield in older adults with fever, headache, jaw claudication, visual symptoms, or high ESR
- Biopsy of the most abnormal accessible tissue is often more useful than repeated broad serologies
Overview
Fever of unknown origin is a syndrome rather than a diagnosis. The classic definition requires prolonged fever without a diagnosis after appropriate evaluation. Modern workup emphasizes pattern recognition, epidemiologic exposures, medication review, immune status, travel, animal exposure, indwelling devices, and repeated physical examination. Stable patients usually benefit from structured evaluation rather than empiric treatment, because antibiotics or corticosteroids can obscure cultures, histology, and inflammatory findings.
Epidemiology
The distribution of FUO causes has shifted with improved imaging and diagnostics. Infections remain important, including endocarditis, abscess, tuberculosis, osteomyelitis, HIV, EBV/CMV, tick-borne disease, and endemic fungi. Malignancies include lymphoma, leukemia, renal cell carcinoma, and hepatocellular carcinoma. Noninfectious inflammatory causes include giant cell arteritis, adult-onset Still disease, SLE, vasculitis, and inflammatory bowel disease. Many cases remain undiagnosed and resolve spontaneously.
Clinical Features
Symptoms
Daily or intermittent fever >38.3 C for weeks with constitutional symptoms
Weight loss, night sweats, pruritus, or lymphadenopathy suggests lymphoma, TB, HIV, or endemic fungi
Headache, jaw claudication, visual symptoms, or polymyalgia symptoms in age >50 suggests giant cell arteritis
New murmur, embolic phenomena, or injection drug use suggests endocarditis
Travel, animal exposure, tick bite, occupational exposure, or unpasteurized dairy may identify uncommon infections
Drug fever: fever without toxicity, relative bradycardia, rash/eosinophilia sometimes; recent new medication
Signs
Repeated full skin, lymph node, oral, cardiac, abdominal, joint, neurologic, and fundoscopic exams may reveal evolving clues
Lymphadenopathy or hepatosplenomegaly suggests malignancy, EBV/CMV, HIV, TB, or histoplasmosis
Temporal artery tenderness, reduced pulse, or visual deficit is an emergency
New murmur, splinter hemorrhages, Janeway lesions, Osler nodes, or Roth spots suggests endocarditis
Synovitis, rash, oral ulcers, serositis, or purpura suggests rheumatologic disease
Investigations
First-line
CBC with differential, CMP, ESR/CRPScreen for cytopenias, liver injury, renal injury, inflammation, eosinophilia, and malignancy clues
Blood culturesMultiple sets before antibiotics, especially if endocarditis, bacteremia, or device infection possible
Urinalysis and urine cultureOccult urinary infection, renal inflammation, hematuria/proteinuria suggesting vasculitis or malignancy
Chest X-rayScreen for pneumonia, TB, malignancy, sarcoidosis, or mediastinal adenopathy
Second-line
HIV Ag/Ab, hepatitis testing, TB testingBased on risk and baseline evaluation; HIV is important because it changes the differential
CT chest/abdomen/pelvisIf initial evaluation unrevealing or symptoms suggest occult abscess, malignancy, lymphadenopathy, or granulomatous disease
EchocardiographyIf murmur, bacteremia, embolic signs, prosthetic valve, injection drug use, or culture-negative endocarditis concern
Specialist
Biopsy of abnormal tissueLymph node, temporal artery, liver, bone marrow, skin, or mass biopsy often provides definitive diagnosis
FDG-PET/CTCan localize occult inflammatory, malignant, or infectious foci when conventional evaluation is unrevealing
1
General principles
- Confirm true fever and document pattern; review home measurements and inpatient vitals
- Stop nonessential medications when drug fever is plausible
- Repeat the physical exam over time; new signs often emerge
- Use epidemiologic clues: travel, animals, occupation, sexual history, injection drug use, implants, immunosuppression
2
Avoid premature empiric therapy
- Stable non-neutropenic FUO: avoid empiric antibiotics or steroids until diagnostic specimens obtained
- Exceptions: hemodynamic instability, neutropenic fever, suspected bacterial meningitis, TB meningitis, temporal arteritis with visual symptoms, or culture-negative endocarditis after cultures
- If corticosteroids are needed for suspected giant cell arteritis, obtain temporal artery biopsy promptly but do not delay steroids for threatened vision
3
Targeted treatment
- Treat based on confirmed source: endocarditis, TB, abscess, osteomyelitis, endemic fungi, rheumatologic disease, or malignancy
- Drain abscesses and remove infected devices when identified
- Involve infectious diseases, rheumatology, hematology/oncology, or surgery depending on leading diagnosis
Complications
- Delayed diagnosis: Occult endocarditis, TB, abscess, lymphoma, or vasculitis can progress
- Diagnostic obscuration: Empiric antibiotics or steroids may sterilize cultures or alter biopsy yield
- Vision loss: Untreated giant cell arteritis can cause irreversible blindness
- Nosocomial harm: Repeated invasive testing and broad antibiotics can cause complications
- Functional decline: Prolonged fever, weight loss, and inflammation can cause frailty
USMLE Step 2 CK Exam Tips
- 1Stable FUO = diagnose first, do not reflexively give broad-spectrum antibiotics
- 2Older adult + FUO + headache/jaw claudication/high ESR = giant cell arteritis; give steroids if vision threatened
- 3FUO + new murmur or embolic signs = endocarditis; get multiple blood cultures
- 4FUO + night sweats + weight loss + lymphadenopathy = biopsy the lymph node, not empiric antibiotics
- 5Drug fever can persist with relative well appearance and resolves after stopping the culprit drug
- 6Travel and animal exposure history is not decoration — it often identifies brucellosis, Q fever, malaria, TB, or endemic fungi
- 7Neutropenic fever is not classic stable FUO; it needs immediate empiric antipseudomonal antibiotics
practicetest your knowledge on fever of unknown originApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — infectious diseases and beyond.
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