Fever of unknown origin is classically defined as a temperature ≥38.3°C (101°F) on several occasions, persisting for ≥3 weeks, without a diagnosis despite a reasonable initial workup. [1-2] Updated proposals suggest a lower threshold of ≥38.0°C and emphasize quality-based criteria (a list of negative investigations) rather than rigid time cutoffs. [2] The four major etiologic categories are infections (~30–50%), non-infectious inflammatory diseases (~5–32%), malignancy (~6–19%), and miscellaneous causes; 13–51% of cases remain undiagnosed. [1][3-4]
1. History
- Duration, pattern, and magnitude of fever: continuous, intermittent, relapsing (Pel-Ebstein pattern suggests lymphoma); regularly intermittent fever may suggest UTI, irregularly intermittent may suggest endocarditis [3]
- Constitutional symptoms: weight loss (malignancy, TB, endocarditis), night sweats, fatigue, anorexia
- Localizing symptoms: cough, dysuria, arthralgia, rash, headache, abdominal pain, back pain, diarrhea
- Exposures: travel history (malaria, enteric fever, leptospirosis), animal contacts (brucellosis, Q fever, leptospirosis), tick/insect bites, unpasteurized dairy, raw meat [1][5]
- Sexual history and IV drug use: HIV seroconversion, endocarditis risk [6]
- Occupational exposures: healthcare workers (TB), farmers (brucellosis), slaughterhouse workers
- Medication history: new drugs in the preceding 7–10 days (beta-lactams, anticonvulsants, allopurinol) [1]
- Dental history: poor dentition raises endocarditis risk [7]
- Surgical/procedural history: recent instrumentation, prosthetic valves, implanted devices
- Important negatives: no recent antibiotics (which may mask cultures), no immunosuppression, no known malignancy
2. Alarm Features
- Hemodynamic instability (sepsis, septic shock)
- New heart murmur with fever → endocarditis until proven otherwise [6-7]
- Focal neurologic deficits → CNS abscess, meningitis, embolic stroke from endocarditis
- Severe cytopenias (pancytopenia) → hematologic malignancy, HLH
- Rapidly progressive weight loss → occult malignancy, disseminated TB
- Petechiae, Janeway lesions, Osler nodes, splinter hemorrhages → endocarditis [7]
- Markedly elevated ferritin (>10,000 ng/mL) → adult-onset Still's disease, HLH [1]
- Rigors with high spiking fevers → bacteremia, abscess, lymphoma
3. Medications
- Common culprits of drug fever: beta-lactams (most common), sulfonamides, vancomycin, anticonvulsants (phenytoin, carbamazepine), allopurinol, heparin [1]
- Hyperthermic syndromes: SSRIs/SNRIs (serotonin syndrome), antipsychotics (neuroleptic malignant syndrome), succinylcholine/inhalational anesthetics (malignant hyperthermia) [1]
- DRESS syndrome: sulfonamides, carbamazepine, allopurinol, lamotrigine — onset 2–6 weeks [1]
- Key clinical pearl: drug fever is a diagnosis of exclusion; eosinophilia present in only ~25%, rash in ~5%; fever typically resolves within 48–72 hours of drug discontinuation [1][8]
- Avoid empiric antibiotics or steroids unless the patient is neutropenic, severely immunocompromised, or critically ill — empiric therapy may delay diagnosis [1][9]
4. Diet
- Exposure-related dietary history: unpasteurized milk/cheese (brucellosis, listeriosis), undercooked meat (toxoplasmosis, enteric fever), raw fish (parasitic infections)
- Hydration: ensure adequate fluid intake during prolonged febrile illness to prevent dehydration
- Whipple's disease: may present with diarrhea, weight loss, and arthralgias — dietary malabsorption is a clue [1]
- No specific dietary intervention treats FUO itself; dietary history is primarily useful for identifying infectious exposures
5. Review of Systems
- General: weight loss, fatigue, night sweats, anorexia
- HEENT: oral ulcers (SLE, Behçet's), temporal headache/jaw claudication (giant cell arteritis), pharyngitis (EBV)
- Cardiovascular: new dyspnea, palpitations (endocarditis, atrial myxoma)
- Pulmonary: cough, hemoptysis (TB, PE, lung cancer)
- GI: diarrhea (Whipple's, IBD, enteric fever), abdominal pain (abscess, lymphoma)
- GU: dysuria, flank pain (UTI, renal abscess, prostatitis)
- MSK: arthralgias/arthritis (Still's disease, SLE, reactive arthritis, Whipple's)
- Skin: rash (Still's — evanescent salmon-colored; SLE — malar; vasculitis — palpable purpura)
- Neuro: headache, confusion, focal deficits (CNS infection, vasculitis, emboli)
- Heme/lymph: lymphadenopathy, splenomegaly (lymphoma, EBV, CMV, endocarditis)
6. Collateral History and Family History
- Collateral: confirm fever documentation (factitious fever is a recognized cause), medication compliance, sick contacts, recent hospitalizations
- Family history: periodic fever syndromes (familial Mediterranean fever, Muckle-Wells syndrome), autoimmune diseases (SLE, RA), malignancy [1]
- Social context: housing conditions, incarceration history (TB exposure), immigration from endemic areas, occupational exposures, substance use
7. Risk Factors
8. Differential Diagnosis
The following table from the NEJM review by Haidar and Singh illustrates the breadth of non-infectious causes:
- Infections (most common overall): tuberculosis (most common single infectious cause globally), endocarditis, occult abscess (hepatic, splenic, pelvic, dental), osteomyelitis, UTI, CMV/EBV, HIV, brucellosis, enteric fever [1][5][12]
- Non-infectious inflammatory diseases: adult-onset Still's disease (younger patients), giant cell arteritis/polymyalgia rheumatica (elderly), SLE, vasculitis, sarcoidosis, IBD [1][12]
- Malignancy: lymphoma (most common malignant cause, especially non-Hodgkin), leukemia, renal cell carcinoma, hepatocellular carcinoma [1][12]
- Miscellaneous: drug fever, PE/DVT, factitious fever, thyroiditis, adrenal insufficiency, atrial myxoma, hemophagocytic lymphohistiocytosis [1]
- Cannot-miss diagnoses: infective endocarditis, disseminated TB, occult lymphoma, deep abscess, PE
9. Past Medical History
- Prior episodes of prolonged fever (periodic fever syndromes, relapsing infections)
- Valvular heart disease, prosthetic valves, cardiac devices → endocarditis risk [6]
- Prior TB exposure or positive PPD/IGRA → reactivation TB
- Autoimmune disease history → disease flare vs. infection on immunosuppression
- Prior malignancy → recurrence, treatment-related immunosuppression
- Surgical history: splenectomy (encapsulated organisms), recent procedures (abscess, seeding)
- Chronic liver/kidney disease: altered immune function, drug clearance
10. Physical Exam
- Vital signs: temperature pattern documentation, relative bradycardia (suggests drug fever, intracellular pathogens like typhoid, or factitious fever); tachycardia out of proportion to fever (PE, sepsis)
- Skin: rash (evanescent salmon rash of Still's, malar rash of SLE, erythema nodosum), petechiae, Janeway lesions, Osler nodes, splinter hemorrhages [7]
- Eyes: conjunctival hemorrhage, Roth spots (endocarditis), uveitis (sarcoidosis, Behçet's)
- Oropharynx: oral ulcers, dental abscess, pharyngeal erythema
- Lymph nodes: generalized vs. localized lymphadenopathy — location and character guide biopsy
- Cardiac: new or changing murmur (endocarditis) [7]
- Abdomen: hepatosplenomegaly (lymphoma, endocarditis, EBV, brucellosis), tenderness (abscess)
- MSK: joint swelling/effusion (Still's, SLE, septic arthritis, Whipple's)
- Rectal/prostate exam: prostatitis, perirectal abscess
- Temporal arteries: tenderness, diminished pulse (giant cell arteritis in patients >50 years)
11. Lab Studies
Minimum initial workup: [13-14]
- CBC with differential
- CMP (electrolytes, renal function, liver enzymes, LDH)
- ESR and CRP
- Blood cultures (≥2 sets from different sites, before antibiotics)
- Urinalysis and urine culture
- HIV testing
- ANA, RF (if autoimmune disease suspected)
- Ferritin (markedly elevated in Still's disease and HLH) [1]
- Procalcitonin (may help distinguish bacterial infection, though limited specificity in FUO) [10][15]
Directed second-line labs based on clinical clues
- T-SPOT.TB or QuantiFERON (tuberculosis) [10]
- Peripheral blood smear (malaria, hematologic malignancy)
- LDH (elevated in lymphoma, HLH, hemolysis)
- CMV/EBV serologies or PCR [1]
- Serum protein electrophoresis (myeloma)
- Thyroid function tests (thyroiditis, thyrotoxicosis)
- Cortisol level (adrenal insufficiency)
Lab clues to etiology: [10]
- Elevated ferritin + leukocytosis + high ESR → non-infectious inflammatory disease
- Low hemoglobin + low platelets + high LDH + high ferritin → malignancy
- Elevated CRP + positive T-SPOT → infectious etiology
- Low/negative procalcitonin → less likely bacterial, consider drug fever [15]
12. Imaging
- First-line: Chest X-ray (all patients); CT chest/abdomen/pelvis if initial labs unrevealing [13-14]
- FDG PET/CT is the imaging modality of choice when initial workup is non-diagnostic:
- Sensitivity 72–98%, specificity 52–85% [1][16]
- Diagnostic yield >50%, at least 30% greater than conventional CT [1]
- Best performance with elevated inflammatory markers and active fever at time of scan [16]
- A negative PET/CT is associated with high likelihood of spontaneous fever remission [1][17]
- Directly influenced clinical management in ~75% of patients in a large multicenter study [16]
- Echocardiography: TTE as initial screen if endocarditis suspected; TEE if TTE negative but clinical suspicion persists [18-19]
- Abdominal/pelvic ultrasound: hepatic/splenic abscess, lymphadenopathy, renal pathology
- MRI: targeted use for osteomyelitis, CNS lesions, spinal epidural abscess
- When imaging is unnecessary: stable, well-appearing patient with self-resolving fever and normal initial labs — observation may be appropriate
13. Special Tests
- Tissue biopsy (highest diagnostic yield among invasive tests): lymph node, liver, bone marrow, temporal artery (age >50 with elevated ESR) [9]
- Bone marrow biopsy: indicated when hematologic malignancy, granulomatous disease, or HLH is suspected
- Temporal artery biopsy: consider in elderly patients with unresolved FUO, even without classic GCA symptoms [1]
- Next-generation sequencing (mNGS): emerging role for culture-negative infections; reserve for cases that remain undiagnosed after standard workup [1][20-21]
- Naproxen test: historical; fever response to naproxen was thought to suggest neoplastic fever, but this is unreliable and not recommended as a definitive diagnostic tool
14. ECG
- Indications: all patients with FUO should have a baseline ECG
- Findings suggesting endocarditis complications: new conduction abnormalities (PR prolongation, new bundle branch block) → perivalvular abscess [18]
- Pericarditis pattern: diffuse ST elevation, PR depression (SLE, TB, viral)
- Sinus tachycardia: expected with fever; absence (relative bradycardia) may suggest drug fever, typhoid fever, or factitious fever
- Atrial fibrillation/flutter: new onset may suggest thyrotoxicosis, PE, or myocarditis
15. Assessment
- FUO is most often an atypical presentation of a common disease rather than a rare disease [9][14]
- Infections remain the leading cause globally, with tuberculosis as the single most common infectious etiology [1][5]
- In high-income settings, non-infectious inflammatory diseases and undiagnosed cases are increasingly prevalent [1][4]
- Up to 51% of cases remain undiagnosed even in the current era; these patients generally have an excellent prognosis with high rates of spontaneous remission [1][9]
- Severity stratification depends on hemodynamic stability, degree of weight loss, cytopenias, and presence of alarm features
- Atypical presentations to consider: elderly patients with GCA may lack headache; endocarditis may present without murmur; disseminated TB may have normal chest X-ray [1][7]
16. Treatment Plan
Initial stabilization
- IV fluids and antipyretics for symptomatic relief
- Hemodynamic support if septic
Empiric therapy — generally avoid unless: [1][9]
- Neutropenic → broad-spectrum antibiotics per neutropenic fever protocols
- Severely immunocompromised → targeted empiric coverage based on host factors
- Critically ill/rapidly deteriorating → empiric broad-spectrum antibiotics after cultures obtained
- Strong clinical suspicion for specific diagnosis (e.g., doxycycline trial for suspected zoonosis, anti-TB therapy for high-probability TB) [1]
Directed therapy once diagnosis established
- Endocarditis → pathogen-directed IV antibiotics per guidelines [18]
- TB → standard 4-drug regimen (RIPE)
- Adult-onset Still's disease → NSAIDs, glucocorticoids, IL-1 receptor antagonists
- Lymphoma → oncology referral for chemotherapy
- Drug fever → discontinue offending agent; fever typically resolves within 48–72 hours [8]
- GCA → high-dose glucocorticoids (do not delay if vision threatened)
Key pearl: empiric steroids should be avoided as they may mask lymphoma, infection, or other treatable diagnoses [1][9]
17. Disposition
- Admission criteria:
- Hemodynamic instability or sepsis
- Severe cytopenias or concern for hematologic emergency
- Need for urgent invasive diagnostics (biopsy, TEE)
- Significant weight loss, failure to thrive, inability to maintain oral intake
- High suspicion for endocarditis, deep abscess, or disseminated TB requiring IV therapy
- Immunocompromised host with FUO
- Outpatient workup appropriate when:
- Hemodynamically stable, well-appearing
- Able to tolerate oral intake and follow up reliably
- No alarm features
- Initial labs do not suggest emergent pathology
- Observation: consider for patients with borderline presentations pending culture results or imaging
- Specialist consultation triggers:
- Infectious disease: culture-negative endocarditis, suspected TB, HIV, complex infections
- Rheumatology: suspected autoimmune/autoinflammatory disease
- Hematology/oncology: cytopenias, lymphadenopathy, suspected malignancy
- Surgery: suspected deep abscess requiring drainage
18. Follow Up / Return Precautions
- Follow-up timing: within 1 week if discharged with pending workup; sooner if cultures or biopsy results expected
- Repeat history and exam: diagnostic clues may emerge on repeat questioning — 48–81% of initial clues may be misleading, and new clues often appear over time [1]
- Symptoms requiring immediate reassessment:
- New rigors, hemodynamic instability, or altered mental status
- New rash, joint swelling, or focal neurologic findings
- Worsening weight loss or functional decline
- New heart murmur or embolic phenomena
- Patient counseling:
- FUO often resolves spontaneously without a definitive diagnosis — up to 75% of cases [9]
- Avoid self-medicating with antibiotics, which may delay diagnosis
- Keep a fever diary documenting temperature, timing, and associated symptoms
- Expected course: patients with undiagnosed FUO who remain stable generally have an excellent prognosis; mortality in hospitalized FUO cohorts is low (~1.9–2.8%) [22-23]
Would you like to focus on a specific FUO subtype (e.g., nosocomial, immunocompromised, travel-associated) or a particular diagnostic pathway such as the PET/CT workup algorithm?
References
1. Fever of Unknown Origin. — Haidar G, Singh N. The New England Journal of Medicine. 2022.
2. Fever of Unknown Origin (FUO) - A Call for New Research Standards and Updated Clinical Management. — Wright WF, Mulders-Manders CM, Auwaerter PG, Bleeker-Rovers CP. The American Journal of Medicine. 2022.
3. Clinical Analysis of 215 Consecutive Cases With Fever of Unknown Origin: A Cohort Study. — Zhai YZ, Chen X, Liu X, et al. Medicine. 2018.
4. FDG PET/CT, C-Reactive Protein, and Charlson Comorbidity Index in Fever of Unknown Origin: A Retrospective Two-Center Cohort Study. — Gonenli MG, Muslu B, Bayramlar OF, et al. International Journal of Infectious Diseases : IJID : Official Publication of the International Society for Infectious Diseases. 2026.
5. Geographic Variation of Infectious Disease Diagnoses Among Patients With Fever of Unknown Origin: A Systematic Review and Meta-Analysis. — Wright WF, Yenokyan G, Simner PJ, Carroll KC, Auwaerter PG. Open Forum Infectious Diseases. 2022.
6. Infective Endocarditis. — Li M, Kim JB, Sastry BKS, Chen M. Lancet. 2024.
7. Native-Valve Infective Endocarditis. — Chambers HF, Bayer AS. The New England Journal of Medicine. 2020.
8. Drug Fever. — Lipsky BA, Hirschmann JV. The Journal of the American Medical Association. 1981.
9. Fever of Unknown Origin in Adults. — David A, Quinlan JD. American Family Physician. 2022.
10. Fever of Unknown Origin: Clinical Significance of the Etiology and Common Inflammatory Parameters. — Xie N, Zhang W, Tian F, et al. Diagnostic Microbiology and Infectious Disease. 2025.
11. Prevalence of Drug Fever Among Cases of Nosocomial Fever: A Systematic Review and Meta-Analysis. — Someko H, Okazaki Y, Kuniyoshi Y, et al. Internal Medicine. 2024.
12. Diagnosis and Outcomes of Fever of Unknown Origin Cases With an Erythrocyte Sedimentation Rate of 100 mm/H or More: An International ID-IRI (Infectious Diseases - International Research Initiative) Observational Retrospective Cohort Study. — Elbahr U, Erdem H, Ben Yahia W, et al. Medicine. 2025.
13. Summary: Appropriate Use Criteria for the Use of Nuclear Medicine in Fever of Unknown Origin. — Palestro CJ, Abikhzer G, Bar-Sever Z, et al. Journal of Nuclear Medicine : Official Publication, Society of Nuclear Medicine. 2024.
14. Prolonged Febrile Illness and Fever of Unknown Origin in Adults. — Hersch EC, Oh RC. American Family Physician. 2014.
15. A Retrospective Analysis of Drug Fever Diagnosed During Infectious Disease Consultation. — Yaita K, Sakai Y, Masunaga K, Watanabe H. Internal Medicine. 2016.
16. Diagnostic Yield of [18F]FDG PET/CT in FUO: An Italian Multicenter Study of 929 Patients. — Albano D, Lanfranchi F, Bauckneht M, et al. Journal of Nuclear Medicine : Official Publication, Society of Nuclear Medicine. 2026.
17. The Value of [18F]FDG PET/CT in Fever of Unknown Origin (FUO): An Update and Future Aspects. — Albano D, Urso L, Treglia G, Bertagna F. Seminars in Nuclear Medicine. 2026.
18. Infective Endocarditis in Adults: Diagnosis, Antimicrobial Therapy, and Management of Complications: A Scientific Statement for Healthcare Professionals From the American Heart Association. — Baddour LM, Wilson WR, Bayer AS, et al. Circulation. 2015.
19. Diagnosis and Management of Infective Endocarditis in People Who Inject Drugs: JACC State-of-the-Art Review. — Yucel E, Bearnot B, Paras ML, et al. Journal of the American College of Cardiology. 2022.
20. Progress Report: Next-Generation Sequencing, Multiplex Polymerase Chain Reaction, and Broad-Range Molecular Assays as Diagnostic Tools for Fever of Unknown Origin Investigations in Adults. — Wright WF, Simner PJ, Carroll KC, Auwaerter PG. Clinical Infectious Diseases : An Official Publication of the Infectious Diseases Society of America. 2022.
21. Fever and Inflammation of Unknown Origin in the 21st Century. — Antoniadou C, Gavriilidis E, Chatzopoulos P, Gkouliavera M, Skendros P. European Journal of Internal Medicine. 2025.
22. Structured Diagnostic Scheme Clinical Experience Sharing: A Prospective Study of 320 Cases of Fever of Unknown Origin in a Tertiary Hospital in North China. — Jiang L, Wu H, Zhao S, Zhang Y, Song N. BMC Infectious Diseases. 2023.
23. Fever of Unknown Origin (FUO) FADOI-SIMIT Italian Registry: Can Demographics, Comorbidities, and Clinical Variables Predict the Etiology of Classic FUO?-a Prospective Italian Study. — Luzzati R, Zerbato V, Attard L, et al. Internal and Emergency Medicine. 2025.