Fever of unknown origin

This page is for adult patients. For pediatric patients, see: fever of unknown origin (peds)

Background

  • Classic FUO definition: Temperature ≥38.3°C (101°F) on multiple occasions, duration ≥3 weeks, without an established diagnosis after a thorough initial evaluation[1]
  • The modern definition emphasizes completion of a minimum obligatory workup rather than a fixed time in hospital[2]
  • FUO is most commonly an atypical presentation of a common disease, not a rare disease[3]
  • Approximate etiologic distribution in developed countries:[3][2]
    • Infections: ~20-30%
    • Non-infectious inflammatory diseases (NIID): ~20-30% (increasingly the most common category in developed countries)
    • Malignancy: ~10-15%
    • Miscellaneous: ~10-15%
    • Undiagnosed: ~20-30% (prognosis is generally favorable; most resolve spontaneously)

FUO Subcategories

Category Definition
Classic FUO Immunocompetent, community-acquired; as defined above
Nosocomial FUO Hospitalized ≥24 hrs without fever on admission; fever ≥38.3°C on multiple occasions; no identified source after 3 days of workup
Neutropenic FUO Neutrophil count <500/µL; fever ≥38.3°C on multiple occasions; no identified source after 3 days (see Febrile neutropenia)
HIV-related FUO Confirmed HIV infection; fever ≥38.3°C on multiple occasions for >4 weeks (outpatient) or >3 days (inpatient)
  • True classic FUO is rarely diagnosed in the ED — most ED patients with unexplained fever have an acute undifferentiated febrile illness of shorter duration
  • The ED role is to:
    • Identify and treat life-threatening causes (sepsis, endocarditis, malignancy, MAS/HLH)
    • Initiate the workup framework (labs, cultures, imaging)
    • Determine safe disposition (admit vs. outpatient workup with close follow-up)
    • Recognize patterns that suggest specific diagnoses (see Clinical Features)
  • Avoid empiric antibiotics in hemodynamically stable FUO patients without a suspected source — this can mask infection, delay diagnosis, and confuse cultures[3]
    • Exception: empiric antibiotics are appropriate if the patient is septic, neutropenic, or immunocompromised

Clinical Features

A thorough history and physical exam is the single most important diagnostic tool in FUO. Potential diagnostic clues (PDCs) guide targeted testing.[2]

History — Key Questions

  • Fever pattern:
  • Travel history: Malaria, typhoid, brucellosis, histoplasmosis, coccidioidomycosis, leishmaniasis, tuberculosis
  • Animal/zoonotic exposure: Cat scratch disease (Bartonella), Q fever (farm animals), brucellosis (unpasteurized dairy), leptospirosis, tularemia
  • Medications: Drug fever is a diagnosis of exclusion (see Differential Diagnosis); any medication can cause fever; beta-lactams, anticonvulsants, and allopurinol are common culprits
  • Occupational exposures: Healthcare workers, laboratory workers, farmers
  • Sexual history: HIV, gonococcal endocarditis, syphilis
  • IV drug use: Endocarditis, osteomyelitis, hepatitis, HIV
  • Immunosuppression: Transplant, chemotherapy, biologics, steroids, HIV → opportunistic infections, lymphoma
  • Dental history: Dental abscess, endocarditis risk
  • Family history: Periodic fever syndromes, malignancy, autoimmune disease
  • Implanted devices/prosthetics: Infected pacemaker leads, prosthetic joints, vascular grafts

Physical Exam — Focused on High-Yield Findings

Finding Consider
New heart murmur, splinter hemorrhages, Janeway lesions, Osler nodes Endocarditis
Lymphadenopathy (diffuse) Lymphoma, HIV, SLE, Adult-onset Still's disease, sarcoidosis, infections (EBV, CMV, TB, cat scratch)
Hepatomegaly and/or splenomegaly Lymphoma, leukemia, endocarditis, AOSD, SLE, brucellosis, malaria, MAS/HLH
Evanescent salmon-colored rash Adult-onset Still's disease
Tender temporal arteries, jaw claudication Giant cell arteritis (especially age >50)
Oral ulcers SLE, Behçet disease, PFAPA (peds)
Skin nodules, ulcers, or embolic lesions Vasculitis, endocarditis, mycobacterial infection, deep fungal infection
Spinal/paraspinal tenderness Osteomyelitis, epidural abscess, discitis
Testicular mass or tenderness Lymphoma, testicular cancer, polyarteritis nodosa
Dental caries/periodontal disease Dental abscess, endocarditis risk
Fundoscopic abnormalities (Roth spots, choroid tubercles) Endocarditis, disseminated TB, CMV retinitis (HIV)
Rectal/pelvic exam abnormalities Pelvic abscess, prostatitis, rectal malignancy
  • ED Pearl: Up to 97% of FUO patients have identifiable findings on history or exam, but these lead to a diagnosis in only ~62% — repeated and careful examination over time is essential[3]
  • Rule out factitious fever (reported in up to 9% of FUO cases): suspect when fever >41°C without tachycardia or skin warmth, temperature documented only by patient, healthcare worker with prolonged FUO, or atypical fever patterns[3]

Differential Diagnosis

Infections (~30%)

Malignancy (~20%)

  • Hematologic (most common malignant cause of FUO)
    • Lymphoma (Hodgkin and non-Hodgkin)
    • Leukemia (especially acute leukemia)
    • Multiple myeloma
    • Myelodysplastic syndrome
    • Castleman disease
  • Solid Tumors
    • Renal cell carcinoma
    • Hepatocellular carcinoma
    • Colon cancer
    • Pancreatic cancer
    • Atrial myxoma
    • Pheochromocytoma

Autoimmune/Inflammatory (~20%)

Drug Fever

Endocrine/Metabolic

Thromboembolic/Vascular

Factitious/Habitual

  • Factitious fever (self-induced)
  • Munchausen syndrome / Munchausen by proxy (pediatric)

Miscellaneous

  • Post-surgical/post-procedural inflammation
  • Gout/pseudogout (crystal arthropathy)
  • Cirrhosis / alcoholic hepatitis
  • Hemolytic anemia
  • Transfusion reaction
  • Tissue necrosis (rhabdomyolysis, large hematoma, pancreatitis)
  • Hypothalamic dysfunction (central fever)
  • Periodic fever of unknown cause (~10-15% of FUO never diagnosed)

Evaluation

Workup

The approach to FUO is guided by potential diagnostic clues (PDCs) from history and exam. If PDCs are present, pursue targeted testing. If no PDCs, follow a standardized workup.[2][3]

Initial Workup (ED and Early Inpatient)

Laboratories
  • CBC with differential (leukocytosis, left shift, eosinophilia, cytopenias, atypical lymphocytes)
  • ESR, CRP (elevated in most causes of FUO; ESR >100 mm/hr narrows differential to endocarditis, abscess, TB, malignancy, temporal arteritis)
  • Ferritin — markedly elevated in Adult-onset Still's disease, macrophage activation syndrome/HLH; ferritin >5× normal significantly narrows the differential
  • LFTs (AST, ALT, ALP, bilirubin, albumin, LDH)
  • BMP (renal function, electrolytes)
  • Blood cultures — at least 3 sets (each set = aerobic + anaerobic bottle, each with 10 mL blood) from separate venipuncture sites[3]
  • Urinalysis with culture
  • Lactate if concern for sepsis
  • Coagulation studies (PT/PTT, fibrinogen, D-dimer) — especially if MAS/HLH or DIC suspected
  • Peripheral blood smear — evaluate for blasts (leukemia), atypical lymphocytes (EBV/CMV), hemophagocytes, schistocytes
  • Procalcitonin — may help distinguish bacterial infection from non-infectious causes (imperfect but can assist)
  • HIV test (4th-generation Ag/Ab)
  • ANA, RF — if negative, makes SLE and RA less likely; if positive, pursue targeted rheumatologic workup
  • Serum protein electrophoresis (SPEP) — screen for myeloma, Castleman disease
  • Tuberculosis testing: QuantiFERON-Gold or T-SPOT (preferred over PPD for ED evaluation)
Imaging
  • CXR — baseline for infiltrates, lymphadenopathy, pleural effusion, masses
  • CT chest/abdomen/pelvis with IV contrast — if no diagnosis from initial labs; identifies abscess, lymphadenopathy, organomegaly, masses
    • Diagnostic yield: 20-30% in FUO
Other
  • Echocardiography (TTE initially; consider TEE if endocarditis suspected clinically)

Extended Workup (Guided by PDCs or if Initial Workup Non-Diagnostic)

  • FDG-PET/CT — emerging as the preferred advanced imaging modality for FUO when initial workup is non-diagnostic[4][3]
    • Particularly useful for identifying occult infection (abscess, osteomyelitis, infected devices), vasculitis (large vessel), and malignancy (lymphoma)
    • Diagnostic yield: 50-70% in FUO
    • Recommended by AAFP 2022 if ESR or CRP is elevated and initial workup is non-diagnostic[3]
  • Serologies (directed by exposure history):
  • Temporal artery biopsy — adults >50 with elevated ESR, headache, jaw claudication, visual symptoms (giant cell arteritis accounts for 16-17% of FUO in the elderly)
  • Bone marrow biopsy — if concern for hematologic malignancy, granulomatous disease, MAS/HLH, or disseminated infection
  • Lymph node biopsy — excisional preferred over FNA; if significant lymphadenopathy persists
  • Ophthalmologic exam — slit-lamp exam for uveitis/iridocyclitis (sarcoidosis, Behçet's disease, TB)
  • CT sinuses/mastoids — if chronic sinusitis/mastoiditis suspected
  • Colonoscopy — if inflammatory bowel disease suspected (fever, weight loss, abdominal pain, elevated inflammatory markers)
  • Bone scan or MRI — if osteomyelitis suspected
  • LP — if CNS infection, meningeal carcinomatosis, or CNS vasculitis suspected

Diagnosis

  • The diagnosis of FUO is iterative — it often requires serial evaluations over days to weeks
  • The EM physician's role is to initiate the workup, identify life-threatening causes, and ensure appropriate follow-up
  • A structured diagnostic protocol (labs → imaging → tissue biopsy) identifies a cause in more patients than ad hoc testing[2]
  • Key diagnostic patterns:
    • Ferritin >1,000 + negative ANA/RF + quotidian fever + rash + leukocytosis → Adult-onset Still's disease
    • ESR >100 + new murmur + positive blood cultures → endocarditis
    • ESR >100 + age >50 + headache/jaw claudication → Giant cell arteritis
    • Ferritin >10,000 + falling platelets + falling ESR → Macrophage activation syndrome
    • Night sweats + weight loss + lymphadenopathy → Lymphoma or tuberculosis
    • Fever resolves when all medications are stopped → Drug fever

Management

  • Treat the underlying cause once identified
  • Avoid empiric antibiotics in hemodynamically stable patients with classic FUO — they obscure cultures, delay diagnosis, and select for resistant organisms[3]
  • Exceptions to the "no empiric therapy" rule:
    • Hemodynamically unstable / septic: empiric broad-spectrum antibiotics per sepsis guidelines
    • Febrile neutropenia: empiric antipseudomonal beta-lactam within 1 hour (see Febrile neutropenia)
    • Culture-negative endocarditis suspected: empiric endocarditis coverage per guidelines
    • Suspected disseminated tuberculosis: empiric anti-TB therapy if clinical suspicion is high (especially immunocompromised)
    • Temporal arteritis with threatened vision loss: empiric high-dose corticosteroids (do not delay for biopsy)
  • Avoid empiric corticosteroids in undiagnosed FUO — may mask lymphoma, worsen infection, or cause diagnostic confusion[2]
    • Exception: temporal arteritis with visual symptoms (do not wait for biopsy)
  • Antipyretics (acetaminophen, NSAIDs) for symptom control; reasonable in all patients
  • Drug fever management: discontinue all non-essential medications; fever typically resolves within 48-72 hours of stopping the offending agent

Disposition

Admit

  • Hemodynamically unstable or septic
  • Immunocompromised (febrile neutropenia, HIV/AIDS with low CD4, transplant recipients)
  • Evidence of end-organ dysfunction (acute kidney injury, liver failure, coagulopathy)
  • Suspicion for endocarditis, macrophage activation syndrome, or MAS/HLH
  • Suspicion for malignancy requiring urgent biopsy
  • Inability to ensure close outpatient follow-up
  • Significant comorbidities (elderly, debilitated, frail)
  • IV drug users with unexplained fever (high risk for endocarditis, epidural abscess)

Consider Discharge with Close Follow-Up

  • Hemodynamically stable, non-toxic appearing, immunocompetent
  • Initial workup (labs, cultures, imaging) has been initiated
  • No evidence of life-threatening etiology
  • Reliable outpatient follow-up arranged within 2-5 days (primary care, infectious disease, or rheumatology)
  • Clear return precautions: worsening fever, new rash, bleeding, altered mental status, weight loss, new localizing symptoms
  • Document the diagnostic plan — communicate clearly to the follow-up provider which labs are pending and what the next steps are

Prognosis

  • Up to 75% of FUO cases will either be diagnosed or resolve spontaneously without a definitive diagnosis[3]
  • Patients with undiagnosed FUO who remain hemodynamically stable generally have a favorable prognosis[2]
  • FUO lasting >6 months without a diagnosis is most commonly due to habitual hyperthermia, factitious fever, or granulomatous hepatitis — malignancy and serious infection become less likely the longer the fever persists

See Also

External Links

References

  1. ↑ Petersdorf RG, Beeson PB. Fever of unexplained origin: report on 100 cases. Medicine (Baltimore). 1961;40:1-30.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Mulders-Manders C, Simon A, Bleeker-Rovers C. Fever of unknown origin. Clin Med (Lond). 2015;15(3):280-4.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 David A, Quinlan JD. Fever of Unknown Origin in Adults. Am Fam Physician. 2022;105(2):137-143.
  4. ↑ Defined FDG-PET/CT as technique of first choice for FUO without PDCs. Defined FDG-PET/CT role. Defined FDG-PET/CT: FDG-PET/CT is helpful in 50-70% of FUO cases. Defined FDG-PET/CT role in identifying occult infection, vasculitis, and malignancy. Defined role per Defined role.
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