FUO Fundamentals - Defining the Mystery
- Original (Petersdorf & Beeson): Fever >38.3°C, >3 wks; diagnosis unclear post 1 wk inpatient.
- Modified Types (Durack & Street):
- Classical FUO:
⭐ Classical FUO: Fever >38.3°C (101°F) on several occasions, duration >3 weeks, and failure to reach a diagnosis despite 1 week of inpatient investigation or 3 outpatient visits.
- Nosocomial: Hospital-acquired fever >38.3°C; unclear post 3 days.
- Neutropenic: Neutrophils <500/µL, fever >38.3°C; unclear post 3 days.
- HIV-associated: HIV+; fever >38.3°C for >4 wks (outpt) / >3 days (inpt); unclear post 3 days.
- Classical FUO:
FUO Culprits - The Usual Suspects
Common causes of Fever of Unknown Origin (FUO) can be broadly categorized:
| Category | Common Examples (India) | Key Differentiator/Test |
|---|---|---|
| Infections | TB (EPTB, Miliary), Brucellosis, Enteric fever, Abscesses, Endocarditis | AFB, Cultures, Serology, Imaging, Echo |
| Malignancies | Lymphoma, Leukemia, RCC, HCC | Biopsy, BM exam, CT/PET |
| Collagen Vasc./Inflamm. | Adult Still's, SLE, Vasculitis (PAN, GCA) | Ferritin, ANA, ANCA, ESR/CRP, Biopsy |
| Miscellaneous | Drug fever, Factitious fever | Drug withdrawal, Observation |
FUO Detective Work - Cracking the Case
⭐ FDG-PET/CT is highly valuable for localizing occult inflammatory or neoplastic foci in FUO when initial investigations are unrevealing, guiding further targeted biopsies.
FUO Hotspots - Alarms & Alerts
- Elderly: Giant Cell Arteritis (GCA), malignancy.
- Immunocompromised: Opportunistic infections (PJP, CMV, fungal).
- Travel History: Malaria, typhoid, dengue, rickettsial diseases.
- Drug-Induced: Beta-lactams, phenytoin.
- Factitious Fever: Discrepancy oral/rectal temp, no ↑HR.
- Naproxen Test: Lysis suggests neoplastic fever.
⭐ In elderly patients (>65 years) with FUO, headache, and elevated ESR (>50 mm/hr), strongly consider Giant Cell Arteritis (Temporal Arteritis); temporal artery biopsy is the gold standard for diagnosis.
High‑Yield Points - ⚡ Biggest Takeaways
- FUO: Fever > 38.3°C for ≥ 3 weeks, undiagnosed after 1 week inpatient or 3 outpatient visits.
- Main Causes: Infections (TB, endocarditis), malignancies (lymphoma), autoimmune (Still's, temporal arteritis).
- Elderly: ↑ Temporal arteritis, malignancies.
- HIV-FUO: Opportunistic infections (TB, MAC), lymphoma.
- Investigations: Cultures, ESR/CRP, CT abdomen/pelvis.
- Naproxen test: Suggests neoplastic fever; not diagnostic.
- Empirical therapy: Generally avoid unless critically ill/neutropenic_._
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