Fever of Unknown Origin

FUO Definition & Types - Defining the Undefined

• Classic FUO Criteria (Petersdorf & Beeson):
  • Fever > 38.3°C (101°F) on multiple occasions.
  • Duration ≥ 3 weeks.
  • No diagnosis after 1 week of inpatient investigation or 3 outpatient visits.
• Major FUO Categories:
  • Classic
  • Nosocomial (Hospital-acquired)
  • Neutropenic (Immune-deficient; ANC < 500/µL)
  • HIV-associated

⭐ Infections, malignancies, and collagen vascular diseases are the "big three" categories for classic FUO causes. oka

FUO Etiology - The Usual Suspects

The "Big Three" categories account for most FUO cases. Always consider regional epidemiology.

• Infections (~30-50%): Most common cause.

  • Systemic:
    • Tuberculosis (esp. extrapulmonary, miliary)
    • Brucellosis
    • Enteric fever (Typhoid)
    • Infective Endocarditis (culture-negative)
    • HIV (seroconversion, opportunistic infections)
  • Localized:
    • Occult abscesses (intra-abdominal, pelvic, dental)
    • Osteomyelitis
  • Viral: EBV, CMV

• Malignancies (~10-30%):

  • Hematological: Lymphoma (esp. Non-Hodgkin's), Leukemia.
  • Solid tumors: Renal cell carcinoma, Hepatocellular carcinoma, Atrial myxoma.

• Non-infectious Inflammatory Diseases (NIID) / Autoimmune (~10-30%):

  • Adult-onset Still's Disease (AOSD)
  • Systemic Lupus Erythematosus (SLE)
  • Vasculitides (e.g., Giant Cell Arteritis, Polyarteritis Nodosa)
  • Sarcoidosis

• Miscellaneous (~5-15%):

  • Drug fever
  • Factitious fever
  • Thromboembolic disease (DVT/PE)
  • Familial Mediterranean Fever (FMF)

⭐ Tuberculosis (especially extrapulmonary) is a paramount consideration for FUO in the Indian context.

FUO Diagnostics - Detective Work

⭐ FDG-PET/CT can identify the FUO source in up to 50-60% of cases when other tests are unrevealing.

FUO: Indian Context - Desi Dilemmas

• Infections paramount: Tuberculosis (extrapulmonary, miliary), enteric fever (culture-negative), malaria (smear-negative), brucellosis, leptospirosis.
• Endemic focus: Kala-azar (Bihar, Jharkhand), scrub typhus (hilly/rural), dengue, chikungunya.
• Non-infectious mimics: Lymphomas, adult-onset Still's disease, drug fever.
• Key challenge: Indiscriminate empirical anti-TB therapy.

⭐ Extrapulmonary tuberculosis (EPTB) is a leading infectious cause of FUO in India, often with atypical presentations and negative initial workup for pulmonary TB.

FUO Management - Treatment Tactics

• Empirical therapy generally discouraged; await diagnostic clues.
• Exceptions: Neutropenic, critically ill, or rapidly deteriorating patients.
• Tailor treatment to underlying cause once identified.
• Consider NSAIDs for symptomatic relief (cautiously, may mask fever).

⭐ In suspected culture-negative endocarditis as a cause of FUO, empirical treatment often includes vancomycin plus ceftriaxone.

High‑Yield Points - ⚡ Biggest Takeaways

• Classic FUO: Fever > 38.3°C for > 3 weeks, undiagnosed after 1 week of inpatient workup.
• Key causes: Infections (esp. TB, abscesses), malignancies (esp. lymphoma), and autoimmune disorders.
• In India, tuberculosis remains a leading cause.
• Naproxen test may differentiate infectious/inflammatory from neoplastic fevers.
• Elderly FUO: Think giant cell arteritis and polymyalgia rheumatica.
• HIV-associated FUO: Consider opportunistic infections or lymphoma.
• FDG-PET/CT is increasingly used for localizing the fever source.