Fever of Unknown Origin

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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.

Etiology of Fever of Unknown Origin

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 Diagnostic Algorithm

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.

Practice Questions: Fever of Unknown Origin

Test your understanding with these related questions

A person presents to the hospital with fever and chills. Fever profile is ordered and is found to be negative for malaria and dengue. Rk39 test is found to be positive. What is the treatment of choice?

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Flashcards: Fever of Unknown Origin

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The bugs that are associated with Guillain-Barre Syndrome are ICE MyCZ:I_____CMVEBVMycoplasma PneumoniaeCampylobacterZika

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The bugs that are associated with Guillain-Barre Syndrome are ICE MyCZ:I_____CMVEBVMycoplasma PneumoniaeCampylobacterZika

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