Classifications of Pyrexia of Unknown Origin (PUO)
PUO is divided into four distinct subcategories: classical, nosocomial, neutropenic, and HIV-related, each requiring different investigative strategies and having different etiological distributions 1.
The Four Subcategories
1. Classical PUO
- Fever >38.3°C (100.9°F) persisting for at least 3 weeks
- No diagnosis despite 3 outpatient visits or in-patient days 1
- Most common category encountered in general medical practice
- Etiologies include infectious (particularly tuberculosis in endemic regions), inflammatory, malignant, and miscellaneous causes 2
2. Nosocomial (Hospital-Acquired) PUO
- Fever developing in hospitalized patients
- Distinct from classical PUO due to different risk factors and exposures
- Often related to procedures, devices, or healthcare-associated infections 1
3. Neutropenic PUO
- Occurs in patients with neutropenia (typically <500 neutrophils/μL)
- Most commonly seen in oncology and hematology patients
- Carries high mortality risk due to immunocompromised state 1
- Requires urgent evaluation and often empiric antimicrobial therapy
4. HIV-Related PUO
- Fever in patients with known HIV infection
- Different etiological spectrum compared to classical PUO
- Often involves opportunistic infections and HIV-related malignancies 1
Etiological Classification System
Regardless of subcategory, the underlying causes are broadly divided into four major groups 1, 2:
Infectious Causes
- Most common category overall (43-53% of cases in various studies) 3, 4
- Tuberculosis predominates, especially extrapulmonary forms in endemic regions 3
- Other infections include enteric fever, brucellosis, and atypical bacterial infections
Inflammatory/Non-Infectious Causes
- Includes collagen vascular diseases and autoinflammatory conditions (6-20% of cases) 3, 5
- Adult-onset Still's disease is a leading cause in younger adults 3
- Vasculitides such as Takayasu arteritis may present as PUO 6
Neoplastic/Malignant Causes
- Account for 12-21% of cases 3, 4
- Lymphomas (both Hodgkin and non-Hodgkin) are most common 3
- Solid malignancies are less frequent causes
Miscellaneous Causes
- Heterogeneous group including drug fever, factitious fever, and other rare conditions (2-25% of cases) 3, 5
Undiagnosed
- Despite extensive investigation, 12-50% of cases remain undiagnosed 2, 3
- Important caveat: Undiagnosed PUO generally carries a favorable prognosis with spontaneous resolution in many cases 1, 3
Clinical Implications
The distribution of causes varies significantly by PUO subcategory and geographical location 1. In resource-limited settings like South Asia, infectious causes (particularly tuberculosis) predominate, while in developed countries, inflammatory and neoplastic causes are relatively more common 7. This geographic and categorical variation mandates tailored diagnostic approaches rather than universal protocols.
A fifth potential subcategory—elderly PUO—has been proposed 2, recognizing that older patients may have different etiologies and atypical presentations, though this classification is not yet universally adopted in current guidelines.