One-liner#

Systematic approach to prolonged fever (>1 week) and classic fever of unknown origin (FUO: fever >3 weeks, >38.3°C on multiple occasions, no diagnosis after 1 week of investigation), focusing on the major categories—infection, malignancy, autoimmune/inflammatory, and miscellaneous—while avoiding shotgun testing.

Quick nav#

• Red flags / send to ED
• Key history
• Focused exam
• Differential (quick pattern recognition)
• Workup
• Initial management
• Management by diagnosis
• Follow-up
• Patient instructions
• Smartphrase snippets

Red flags / send to ED#

• Hemodynamic instability (hypotension, tachycardia, altered mental status) → ED
• Severe neutropenia (<500 cells/μL) with fever → ED
• New focal neurologic deficits with fever → ED (brain abscess, meningitis)
• Acute severe headache with fever → ED (meningitis, SAH)
• Petechial/purpuric rash → ED (meningococcemia, DIC)
• Severe respiratory distress → ED
• Acute abdomen with fever → ED

Urgent (expedited outpatient workup):

• Fever with significant weight loss (>10%)
• Fever with new lymphadenopathy or hepatosplenomegaly
• Fever with new heart murmur
• Fever in immunocompromised patient
• Fever with night sweats (B symptoms)

Key history#

Define the fever:

• Duration (prolonged = >1 week; classic FUO = >3 weeks)
• Temperature pattern (documented temperatures, not just “feeling hot”)
• Maximum temperature
• Pattern: continuous, intermittent, relapsing
• Response to antipyretics

Classic FUO definition (Petersdorf criteria, modified):

• Fever >38.3°C (101°F) on multiple occasions
• Duration >3 weeks
• No diagnosis after 1 week of intelligent investigation (or 3 outpatient visits)

Categories of FUO (the “big four”):

1. Infection (~25-30%): Abscess, endocarditis, TB, osteomyelitis, occult UTI
2. Malignancy (~15-20%): Lymphoma, leukemia, renal cell carcinoma, hepatocellular carcinoma
3. Autoimmune/inflammatory (~15-25%): Adult-onset Still’s disease, vasculitis, PMR/GCA, SLE, IBD
4. Miscellaneous (~10-15%): Drug fever, factitious fever, DVT/PE, thyroiditis
5. Undiagnosed (~20-30%): Often resolves spontaneously; generally good prognosis

Detailed history (clues are everything):

Infectious clues:

• Travel history (TB, malaria, typhoid, histoplasmosis, coccidioidomycosis)
• Animal exposures (Q fever—cattle/sheep; brucellosis—unpasteurized dairy; cat scratch)
• Tick exposures (Lyme, ehrlichiosis, anaplasmosis, Rocky Mountain spotted fever)
• Sexual history (HIV, syphilis, gonococcal infection)
• IV drug use (endocarditis, hepatitis, HIV)
• Dental work or poor dentition (endocarditis, dental abscess)
• Recent procedures or surgery (abscess, line infection)
• Indwelling devices (catheter, pacemaker, prosthetic joint/valve)
• Occupational exposures (healthcare, veterinary, farming)
• Sick contacts, TB exposure
• Immunocompromised state

Malignancy clues:

• Weight loss, night sweats (B symptoms)
• Lymphadenopathy
• Hepatosplenomegaly
• Bone pain
• Prior malignancy
• Smoking history
• Age >50

Autoimmune/inflammatory clues:

• Joint pain, swelling, morning stiffness
• Rashes
• Oral/genital ulcers
• Eye symptoms (uveitis, scleritis)
• Muscle pain, weakness
• Raynaud’s phenomenon
• Family history of autoimmune disease
• Abdominal pain, diarrhea (IBD)
• Headache, jaw claudication, visual changes (GCA)

Drug fever clues:

• New medication in past 1-4 weeks
• Common culprits: antibiotics (beta-lactams, sulfonamides), anticonvulsants (phenytoin, carbamazepine), allopurinol, procainamide, quinidine
• Patient often looks well despite fever
• May have eosinophilia or rash (but often absent)

Factitious fever clues:

• Healthcare worker or medical knowledge
• Fever only when unobserved
• No tachycardia with fever (pulse-temperature dissociation)
• Dramatic presentation
• Multiple prior evaluations

Medication review:

• All medications (prescription, OTC, supplements)
• Recent changes
• Immunosuppressants

Past medical history:

• Prior malignancy
• Autoimmune conditions
• HIV status
• Splenectomy
• Prosthetic devices

Focused exam#

Vital signs:

• Temperature (confirm fever is real; observe measurement)
• Heart rate (should increase ~10 bpm per 1°C; absence = pulse-temperature dissociation)
• Blood pressure
• Weight (compare to baseline)

General:

• Toxic vs well-appearing
• Cachexia
• Diaphoresis

Skin (examine thoroughly):

• Rashes (vasculitis, Still’s disease, drug reaction)
• Petechiae, purpura
• Embolic phenomena (Janeway lesions, Osler nodes)
• Erythema nodosum
• Livedo reticularis
• Wounds, injection sites

HEENT:

• Conjunctival injection, uveitis
• Oral ulcers
• Dental exam (abscess, poor dentition)
• Sinus tenderness
• Temporal artery tenderness (GCA)
• Fundoscopic exam (Roth spots—endocarditis)

Lymph nodes (all stations):

• Cervical, supraclavicular, axillary, epitrochlear, inguinal
• Size, consistency, tenderness, mobility
• Supraclavicular = high concern for malignancy

Cardiovascular:

• Murmurs (new or changing = endocarditis)
• Splinter hemorrhages

Pulmonary:

• Breath sounds
• Signs of effusion

Abdomen:

• Hepatomegaly, splenomegaly
• Masses
• Tenderness
• Ascites

Musculoskeletal:

• Joint swelling, warmth, tenderness
• Spine tenderness (osteomyelitis, epidural abscess)
• Muscle tenderness

Neurologic:

• Mental status
• Focal deficits
• Meningeal signs

Genitourinary:

• Prostate exam (prostatitis, abscess)
• Testicular exam
• Pelvic exam if indicated

Rectal exam:

• Masses
• Perirectal abscess
• Occult blood

Differential (quick pattern recognition)#

Common/likely (outpatient)#

Can’t-miss / urgent#

Workup#

Approach: Systematic, guided by history and exam—not shotgun testing.

Phase 1: Initial workup (all patients with prolonged fever)

Phase 2: Directed testing based on clinical suspicion

Phase 3: Advanced workup (if still undiagnosed)

PET-CT in FUO:

• Identifies source in 40-70% of cases
• Most useful when ESR/CRP elevated
• Can guide biopsy
• Consider early in workup if high suspicion for occult malignancy or infection

When to refer to Infectious Disease:

• Suspected endocarditis
• Suspected TB
• Positive blood cultures with unclear source
• Immunocompromised patient with FUO
• FUO persisting >4-6 weeks without diagnosis
• Need for complex antimicrobial therapy

When NOT to order extensive workup:

• Patient clinically improving
• Low-grade fever with clear viral prodrome
• Drug fever suspected (stop drug and observe)

Initial management#

General approach:

1. Confirm fever is real (document temperatures)
2. Thorough history and exam (often reveals diagnosis)
3. Systematic workup guided by clinical findings
4. Avoid empiric antibiotics unless patient is unstable or immunocompromised
5. Repeat history and exam—new clues may emerge

Empiric antibiotics:

• Generally avoid in stable patients with FUO—may obscure diagnosis
• Exceptions: hemodynamically unstable, neutropenic, suspected endocarditis

Empiric steroids:

• Avoid until infection and malignancy ruled out
• Exception: strong suspicion for GCA with visual symptoms (risk of blindness)

Supportive care:

• Antipyretics for comfort
• Hydration
• Nutrition

Management by diagnosis#

Drug fever#

Education:

• Fever caused by medication reaction
• Resolves after stopping the drug (usually 48-72 hours)
• May recur if re-exposed

Management:

• Stop suspected medication
• Observe for resolution (fever should resolve in 48-72 hours)
• Document allergy/adverse reaction
• Find alternative medication if needed

Follow-up: 48-72 hours to confirm resolution.

Subacute bacterial endocarditis#

Recognition:

• Prolonged fever (weeks to months)
• Risk factors: abnormal valve, IVDU, poor dentition, recent dental work
• Embolic phenomena: stroke, splenic infarcts, Janeway lesions, Osler nodes
• New or changing murmur

Modified Duke Criteria (simplified):

• Major criteria: Positive blood cultures (typical organisms x2 or persistent); endocardial involvement on echo (vegetation, abscess, new dehiscence, new regurgitation)
• Minor criteria: Predisposing condition or IVDU; fever >38°C; vascular phenomena (emboli, Janeway); immunologic phenomena (Osler nodes, Roth spots, RF+); positive blood cultures not meeting major criteria
• Definite endocarditis: 2 major, OR 1 major + 3 minor, OR 5 minor

PCP role:

• Obtain blood cultures (3 sets from separate sites before antibiotics)
• Order TTE (TEE if TTE negative but suspicion high)
• Refer to cardiology and infectious disease
• Do NOT start empiric antibiotics unless patient unstable—wait for culture results

Referral: Cardiology + ID; hospitalization usually required.

Adult-onset Still’s disease#

Recognition:

• Young adult (16-35 typically)
• Quotidian fever (daily high spikes, often >39°C, returning to normal)
• Evanescent salmon-colored rash (appears with fever)
• Arthralgias or arthritis
• Sore throat
• Lymphadenopathy, hepatosplenomegaly
• Markedly elevated ferritin (often >10,000 ng/mL)
• Leukocytosis with neutrophilia

Diagnosis: Clinical (Yamaguchi criteria); must exclude infection and malignancy.

PCP role:

• Order ferritin (very high is suggestive)
• Rule out infection and malignancy
• Refer to rheumatology

Treatment (rheumatology-directed):

• NSAIDs (mild disease)
• Corticosteroids (moderate-severe)
• DMARDs, biologics (refractory)

Giant cell arteritis (GCA)#

Recognition:

• Age >50 (usually >70)
• New headache (often temporal)
• Scalp tenderness
• Jaw claudication (highly specific)
• Visual symptoms (amaurosis fugax, diplopia, vision loss)
• PMR symptoms (shoulder/hip girdle stiffness)
• Markedly elevated ESR (often >50-100) and CRP

Urgent action: If visual symptoms present, start steroids immediately—do not wait for biopsy (risk of permanent blindness).

Treatment:

Temporal artery biopsy: Should be done within 2 weeks of starting steroids (remains positive).

Referral: Rheumatology for long-term management.

Follow-up: Close monitoring; steroid taper guided by symptoms and inflammatory markers.

Tuberculosis#

Recognition:

• Risk factors: immigration from endemic area, incarceration, HIV, homeless, healthcare worker, known exposure
• Chronic cough (>3 weeks), hemoptysis
• Night sweats, weight loss, fever
• Lymphadenopathy (especially cervical)
• Abnormal CXR (upper lobe infiltrates, cavitation, miliary pattern)

PCP role:

• CXR
• IGRA (QuantiFERON or T-SPOT) or PPD
• If pulmonary TB suspected: sputum AFB smear and culture x3
• Respiratory isolation if active TB suspected
• Report to public health

Referral: Infectious disease or TB clinic; public health notification required.

Do NOT start empiric TB treatment without ID guidance (complex regimens, resistance concerns).

Lymphoma#

Recognition:

• B symptoms: fever, night sweats, weight loss >10%
• Painless lymphadenopathy
• Hepatosplenomegaly
• Elevated LDH
• May have cytopenias or elevated WBC

PCP role:

• CBC, CMP, LDH
• CT chest/abdomen/pelvis
• Refer for lymph node biopsy (excisional preferred)
• Refer to hematology/oncology

Do NOT delay referral—lymphoma is treatable but needs prompt diagnosis.

Undiagnosed FUO#

Prognosis: Generally good if no diagnosis after thorough workup.

• Many resolve spontaneously
• Mortality low in undiagnosed FUO
• May eventually declare itself

Management:

• Close follow-up
• Repeat history and exam periodically
• Avoid repeated extensive testing without new findings
• Symptomatic treatment (antipyretics)
• Consider empiric steroid trial only if autoimmune/inflammatory strongly suspected and infection/malignancy excluded

Follow-up: Every 2-4 weeks initially; less frequent if stable.

Follow-up#

During workup:

• Weekly or more frequent depending on clinical status
• Review results as they return
• Repeat exam—new findings may emerge

After diagnosis:

• Per specific condition
• Coordinate with specialists

Undiagnosed FUO:

• Every 2-4 weeks initially
• Monthly once stable
• Instruct patient to return if new symptoms

Return precautions:

• Worsening fever or new symptoms
• Weight loss
• New lumps or masses
• Visual changes, severe headache
• Bleeding, bruising
• Inability to eat or drink

Patient instructions#

• Prolonged fever can have many causes, and sometimes it takes time to find the answer. We’re working through this systematically.
• Keep a fever diary: record your temperature twice daily (morning and evening) and note any patterns or associated symptoms.
• Take acetaminophen or ibuprofen for comfort if needed.
• Stay well-hydrated and try to maintain good nutrition.
• Avoid starting any new medications or supplements without checking with us first.
• If you’ve been prescribed any new medications recently, let us know—some medications can cause fever.
• Call us right away if you develop: severe headache with neck stiffness, rash (especially if it doesn’t blanch when pressed), confusion, difficulty breathing, or if you feel much worse.
• Keep all follow-up appointments so we can monitor your progress and adjust our plan.

Smartphrase snippets#

.FUOEVAL Evaluation for fever of unknown origin. Patient reports fever for [X weeks]. Maximum documented temperature [X°F/°C]. Pattern: [continuous/intermittent/quotidian]. Associated symptoms: [list]. Exposures: [travel/animals/sick contacts/none]. Medications reviewed. PMH notable for [relevant history]. Exam: [findings]. Initial workup: CBC, CMP, ESR, CRP, LDH, UA, blood cultures x3, CXR, HIV. [Additional directed testing]. Will follow up in [1 week] to review results and reassess.

.FUOWORKUP FUO workup in progress. Initial labs: [results]. CXR: [findings]. Blood cultures: [pending/negative/positive]. [Additional testing ordered/results]. Current differential includes [top considerations]. Plan: [next steps—CT, echo, specialty referral, etc.]. Patient advised to continue fever diary and return if worsening. Follow-up in [timeframe].

.FUODRUGFEVER Suspected drug fever. Patient developed fever [X days/weeks] after starting [medication]. No localizing symptoms. Exam unremarkable. Labs [normal/show eosinophilia]. Most likely drug fever given temporal relationship and absence of other source. Discontinuing [medication]. Expect fever resolution in 48-72 hours. Will follow up to confirm resolution. If fever persists, will expand workup.

.FUOUNDIAGNOSED FUO remains undiagnosed after [X weeks] of evaluation. Workup to date: [summarize key negative results]. Patient clinically [stable/improved/unchanged]. Given negative extensive workup and [stable clinical status/spontaneous improvement], will continue close observation. Differential includes [remaining considerations]. Plan: Symptomatic management; repeat exam in [2-4 weeks]; return sooner if new symptoms or worsening. Prognosis for undiagnosed FUO is generally favorable.

Coding/billing notes#

• R50.9: Fever, unspecified
• R50.81: Fever presenting with conditions classified elsewhere
• I33.0: Acute and subacute infective endocarditis
• M06.1: Adult-onset Still’s disease
• M31.6: Giant cell arteritis (other)
• A15.0: Tuberculosis of lung
• C85.90: Non-Hodgkin lymphoma, unspecified
• C91.00: Acute lymphoblastic leukemia
• D89.9: Disorder involving immune mechanism, unspecified (if autoimmune suspected)