FUO Workup — robbie.med

[FUO] workup v2 · clinical decision support

robbie.med · educational only · not for sole clinical decisions

01 FUO criteria

Temperature documented >38.3°C (101°F)?

Yes No / not documented

Duration of fevers

<3 weeks ≥3 weeks

Setting

Classic Nosocomial Neutropenic HIV-assoc

02 Demographics

Age

Sex

03 Geography & travel

Resides in

Primary region of residence — determines endemic baseline risk

Recent travel (past 12 months)

Mexico / Central Am S America Caribbean Sub-Saharan Africa MENA S Asia SE Asia Mediterranean SW US / desert

04 Exposures

Animal / environmental

Cattle / sheep / goats Raw milk / cheese Undercooked meat Rabbit / wild game Tick exposure / bite Mosquito (endemic) Cats / kittens Cat litter / soil Birds / bats / poultry Caves / spelunking Demolition / construction River valley / Miss-Ohio Freshwater swim Deer / hunting Raw shellfish (oysters)

Healthcare / procedural

Recent surgery (<3 mo) Indwelling catheter / line Prosthetic valve / joint Recent dental work IV drug use Recent transfusion Immunosuppressed HIV / known Transplant New med (3–6 wk)

Social / sexual / TB risk

New sexual partners / MSM Commercial sex TB contact / endemic origin Prison / shelter Homelessness Heavy alcohol use Healthcare worker HBV / HCV Known cirrhosis

05 Clinical features

History / B-symptoms

Weight loss >10% Night sweats Drenching sweats Pruritus (esp post-bath) Periodic / regular pattern Quotidian spikes back to baseline Alcohol-induced LN pain Early anorexia FHx periodic fevers

Head / vision

New headache (esp >50yo) Jaw claudication Scalp / temporal tenderness Vision change / amaurosis Uveitis / eye pain Facial / sinus pain Dental pain / poor dentition Sore throat

Cardiac / pulm / vascular

New murmur Embolic phenomena Splinter / Janeway / Osler Dyspnea Cough >3 wk Hemoptysis Pleuritic chest pain Unilateral leg swelling Atypical PNA pattern

Skin / lymph / MSK

Rash wrists/ankles → central Salmon / evanescent rash Erythema nodosum Malar rash / photosensitive Oral & genital ulcers Morbilliform drug-type rash Localized lymphadenopathy Generalized lymphadenopathy Splenomegaly Hepatomegaly Arthralgia Frank arthritis Proximal stiffness (shldr/hip) Myalgia Sacroiliitis / back pain Bone pain

GI / GU / neuro

Abdominal pain RUQ pain Chronic diarrhea Jaundice Ascites Hematuria Sterile pyuria Flank pain Mononeuritis multiplex Conjunctival suffusion

06 Lab clues

Hematologic

Leukocytosis Leukopenia ANC <500 Eosinophilia Atypical lymphocytes Blasts / immature cells Anemia Hemolytic anemia Thrombocytopenia Thrombocytosis Pancytopenia Polycythemia

Chemistry / acute phase

Transaminitis Elevated alk phos / GGT Hyponatremia Hypercalcemia Elevated LDH ESR >50 ESR >100 CRP elevated Ferritin >500 Ferritin >10,000 Procalcitonin high Elevated CK

UA / other

Pyuria, neg cultures Hematuria on UA Proteinuria Bilateral hilar LAD on CXR Relative bradycardia Patient relatively well-appearing

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A Red flags & empiric Rx considerations

B Stage 1 — initial workup checklist

C Differential — ranked by patient features

All 0 Infection 0 Malignancy 0 Inflammatory 0 Misc 0

D Stage 2 — if no clue after stage 1

E Stage 3 — persistent FUO, advanced workup

F Don't do this

G References & guidance

FUO framework

Wright & Auwaerter — Fever and FUO: Review, Recent Advances, and Lingering Dogma. Open Forum Infect Dis 2020 ↗
Bleeker-Rovers et al. — A prospective multicenter study on FUO: the yield of a structured diagnostic protocol. Medicine 2007 ↗
AAFP — Fever of Unknown Origin in Adults. Am Fam Physician 2022 ↗

CDC clinical guidance (HCP)

Tuberculosis ↗
Malaria ↗
Rocky Mountain Spotted Fever ↗
Ehrlichiosis ↗
Lyme disease ↗
Tularemia ↗
Q fever ↗
Brucellosis ↗
Histoplasmosis ↗
Blastomycosis ↗
Coccidioidomycosis (Valley Fever) ↗

Society guidelines

IDSA — All Practice Guidelines ↗
IDSA/AHA — Infective Endocarditis ↗
ACR — GCA / Takayasu Vasculitis Guideline ↗
ACR — All Clinical Practice Guidelines ↗

Educational support only — not a substitute for clinical judgment. Links open external sites; verify currency before clinical use.

[ robbie.med · FUO Workup v2 · educational support, not a substitute for clinical judgment · feedback welcome ]

[FUO] teaching mode 15-min inpatient block · Internal medicine residents · Academic hospital

1.5 min · Definition 2.5 min · Epidemiology 3 min · History & exam 3 min · Staged workup 2 min · Don't do 2 min · Empiric Rx 1 min · Pearls

01 Definition & subtypes 1.5 min

Discuss first

"What actually makes a fever 'unknown origin'? What's the minimum workup threshold before it qualifies as FUO — and why does the 3-week mark matter clinically?"

Classic FUO: Temperature >38.3°C (>101°F) documented on ≥2 occasions, duration >3 weeks, no diagnosis after adequate evaluation — defined as ≥3 outpatient visits, ≥3 inpatient days, or ≥1 week of structured ambulatory investigation.

The 3-week threshold excludes self-limited viral illness. The "adequate evaluation" clause is the real key — it's not FUO if nobody looked carefully. (Petersdorf & Beeson, 1961; updated Durack & Street, 1991)

Nosocomial

Fever >3 days in hospital, afebrile on admission. Top considerations: drug fever, C. diff, DVT/PE, line infection, occult abscess. Different workup entirely.

Neutropenic — emergency

ANC <500. Antipseudomonal antibiotics within 1 hour. This is a protocol, not a judgment call. Mortality increases with every hour of delay.

HIV-associated

Fever >4 weeks (or >3 days if hospitalized). Expand the differential: MAC, CMV, disseminated histo/cocci, PJP, lymphoma, IRIS.

02 Epidemiology — what's actually causing it? 2.5 min

Discuss first

"Before you look at the chart — just from the population — what's your prior? Infection, malignancy, or inflammatory disease? What shifts your prior most?"

11–55%

Infection

TB, endocarditis, abscess
Brucellosis, Q fever
EBV, CMV, acute HIV
Regional: RMSF, histo, blasto

12–34%

Inflammatory / rheumatic

Adult Still's disease
Giant cell arteritis (≥50)
SLE, ANCA vasculitis
Sarcoidosis, IgG4-RD

7–35%

Malignancy

Hodgkin & NHL (most common)
Leukemia
RCC, HCC, atrial myxoma

2–20%

Miscellaneous

Drug fever
VTE, IBD
Periodic fever syndromes
Factitious fever

7–51%never reach a diagnosis

~40%of undiagnosed cases resolve spontaneously

~7%5-year mortality — mostly NHL

Always on the list — missed too often to ever drop

TBEndocarditisHodgkin / NHLOccult abscessGCA (age ≥50)Drug feverVTE

Trend to know

In modern US academic centers, inflammatory causes have increased and infectious causes have declined — better empiric antibiotics, better imaging, changed epidemiology. If the patient is >65 with weeks of fever and no localizing signs, GCA and Still's move near the top before any other testing.

Wright & Auwaerter, Open Forum Infect Dis 2020 (PMID 32462043) · Bleeker-Rovers et al., Medicine 2007 (PMID 17220753)

03 History & exam — where diagnoses hide 3 min

Discuss first

"Your intern wants to order 'the FUO panel' — ESR, ANA, ANCA, SPEP, ferritin, all serologies at once. What do you say, and why does the order of operations matter?"

"The history is the answer. The labs are the confirmation."

A single exposure detail — cat scratch, raw milk, tick bite, new lisinopril, demolition site — can collapse a 68-disease differential in seconds. No lab panel can do that.

History domains to interrogate every time

Travel (past 12 months) Animal contact Tick / mosquito exposure Raw foods / milk / shellfish New medications (3–6 wk) Sexual history / STI risk TB contacts / incarceration Occupation / demolition / caves River valley / freshwater exposure Alcohol / liver disease Implants / prosthetics / recent dental Family history periodic fevers

Physical exam — what gets missed

Skin in full light: Janeway lesions (painless palmar macules = endocarditis), Osler nodes (painful fingertip nodules), splinter hemorrhages, salmon/evanescent rash (Still's), petechiae (RMSF, meningococcemia), erythema nodosum
All lymph node chains: posterior cervical, supraclavicular, epitrochlear, popliteal — not just groin and axilla
Temporal arteries in any patient ≥50 with headache: tenderness, induration, absent pulse, scalp tenderness
Inside the mouth: dental abscess, gingival findings — often the culprit when present
Fundoscope: Roth spots (endocarditis), choroidal lesions (disseminated fungal / miliary TB)

Fever pattern — don't suppress it

Scheduled antipyretics erase a diagnostic signal. Quotidian (spikes returning to baseline daily) = Adult Still's. Pel-Ebstein (days of fever / days of afebrile, cycling) = Hodgkin. Tertian (q48h) = P. vivax / falciparum. Undulant = brucellosis. Drenching sweats at peak = lymphoma or TB.

04 Staged workup — following the PDC 3 min

PDC = Potentially Diagnostic Clue — any history finding, physical finding, or lab result that narrows the differential. The entire strategy is: find a PDC, follow it with targeted testing, reassess. Do not order everything at once.

Stage 1 — Initial evaluation

CBC with diff, CMP, LFTs, LDH, ferritin, ESR, CRP, UA + culture
Blood cultures ×3 from separate sites — BEFORE any antibiotics
CXR; CT chest if abnormal or high suspicion
CT abdomen/pelvis with IV contrast
HIV 4th-gen Ag/Ab, IGRA (QuantiFERON), EBV/CMV serology

Stage 2 — If no PDC after Stage 1

Targeted serology based on exposure history (Brucella, Bartonella/Coxiella, Lyme two-tier, etc.)
ANA, ANCA, RF, anti-dsDNA, complement if inflammatory suspected
SPEP + UPEP if malignancy / plasma cell disorder suspected
TTE → TEE if endocarditis remains on list despite negative blood cultures
Stop non-essential medications × 72 h to test for drug fever

Stage 3 — Persistent FUO, advanced workup

FDG-PET/CT — highest single-test yield after Stage 1 fails; identifies metabolically active tissue (inflammation, malignancy, occult abscess) and guides biopsy site
Lymph node biopsy: excisional preferred. High yield: posterior cervical, supraclavicular, epitrochlear. Avoid: inguinal, axillary (low specificity, reactive changes)
Temporal artery biopsy in any patient ≥50 with elevated ESR
Bone marrow biopsy (pancytopenia, suspected miliary TB, culture-negative endocarditis)
Liver biopsy if granulomatous hepatitis suspected (elevated alk phos, hepatomegaly)

PET-CT pearl

FDG-PET is the most useful advanced imaging in FUO — not just CT or MRI alone. Best evidence: use it when CRP/ESR are elevated and no localizing clue found after Stage 1–2 workup. It detects occult lymphoma, large-vessel vasculitis (e.g., aortitis in GCA), active granulomas, and abscess that may be invisible on standard cross-sectional imaging.

05 What NOT to do — and why 2 min

Discuss first

"Your patient spikes to 39.9°C at 2 AM. The nurse calls asking for Tylenol orders. What do you say — and if the answer is 'no,' how do you explain it to the patient?"

✕ Empiric broad-spectrum antibiotics for undiagnosed FUO

Sterilizes blood cultures. Masks fever pattern. Creates false reassurance of "responding to treatment." The only exceptions are genuine emergencies where delay is life-threatening — neutropenic fever, suspected RMSF, GCA with visual loss (see Section 6).

✕ Empiric glucocorticoids before a diagnosis

Steroids lower fever in infection, malignancy, and inflammatory disease alike — they are not a diagnostic test. Worse: steroids can lyse lymphoma cells and dramatically reduce biopsy yield. Exception: GCA with active visual symptoms — treat now, biopsy within 1–2 weeks — histopathology remains interpretable.

✕ Scheduled antipyretics masking fever pattern

Quotidian = Still's. Pel-Ebstein = Hodgkin. Tertian = malaria. Undulant = brucellosis. Scheduled Tylenol erases these signals. Use only PRN for comfort at very high temperatures.

✕ Anchoring on a nonspecific positive test

ANA 1:80, positive monospot, mildly elevated ferritin — these are clues, not diagnoses. Premature closure on a low-specificity result causes anchoring and delays the real diagnosis. Continue the systematic evaluation.

✕ Skip the mouth and the skin

Dental abscess, Janeway lesions, Osler nodes, splinter hemorrhages, salmon rash, petechiae — the diagnosis is sometimes visible if you look. 60 seconds, every time.

06 When to treat empirically — the exceptions 2 min

Discuss first

"67-year-old from Oklahoma, 9 days of fever, headache, thrombocytopenia, and transaminitis. No rash yet. Walk me through your decision about empiric treatment."

🔴 Neutropenic fever (ANC <500)

This is a protocol, not a clinical judgment. Antipseudomonal beta-lactam (cefepime, pip-tazo, meropenem based on MASCC risk) within 1 hour. Start before results are back.

🔴 GCA with threatened vision

Amaurosis fugax, diplopia, or monocular visual loss in a patient ≥50 with elevated ESR → prednisone 60 mg PO (or methylprednisolone 1 g IV × 3) today. Do not wait for biopsy. Temporal artery biopsy remains interpretable for 1–2 weeks after steroids are started.

⚠ RMSF / tick-borne — South-Central US, tick exposure

Tick exposure + fever + thrombocytopenia + transaminitis (± headache) in OK/AR/MO/KS/TN → doxycycline 100 mg PO/IV BID now. Serology is negative in the first 7–10 days. Mortality climbs sharply after day 5 — you cannot wait for confirmatory results.

⚠ Miliary / disseminated TB — deteriorating immunosuppressed patient

Immunosuppressed, weight loss + night sweats + bilateral infiltrates, clinical deterioration → empiric RIPE before AFB culture (6–8 weeks). Send sputum AFB ×3, urine AFB, blood AFB, bone marrow culture first — then treat.

⚠ HLH — ferritin >10,000 + cytopenias + splenomegaly

This is HLH until proven otherwise. Immediate hematology + rheumatology co-management. Add triglycerides, fibrinogen, NK activity, sIL-2R. Do not treat as refractory sepsis without excluding HLH — it will not respond and the delay is fatal.

07 Take-home pearls 1 min

FUO is almost always an atypical presentation of a common disease — TB without pulmonary symptoms, endocarditis without a murmur, lymphoma without palpable LAD. Think common diseases presenting uncommonly.
40% of FUO cases never get a diagnosis and many of those resolve spontaneously. An undiagnosed FUO is not automatically a death sentence.
History first, labs second. A single exposure detail collapses the differential instantly. No shotgun panel can do that — and shotgun panels generate false positives that anchor thinking.
Ferritin >10,000 is approximately 90% sensitive for HLH in adults with systemic inflammation. Always check it in FUO; it also points toward Still's disease and lymphoma.
Serial physical exams reveal findings that a single admission exam misses — new lymph node, evolving murmur, temporal artery tenderness. Revisit the exam each day.
The naproxen test: 3 days of naproxen reliably suppress fever → malignant etiology more likely than infectious. Low evidence, but a useful gestalt adjunct.
Lymph node biopsy site matters: posterior cervical, supraclavicular, epitrochlear = high diagnostic yield. Inguinal, axillary = avoid if possible (reactive changes, low specificity).
When FUO persists for months without a diagnosis, revisit the medication list — drug fever can come from an agent started years ago (e.g., hydralazine, phenytoin, allopurinol). Also consider factitious fever in healthcare workers.

Wright & Auwaerter, Open Forum Infect Dis 2020 · Bleeker-Rovers et al., Medicine 2007 · AAFP Am Fam Physician 2022 · Harrison's 20th ed. Educational only · robbie.med