Clinical Evaluation and Management of Pyrexia of Unknown Origin (PUO)

Introduction

Pyrexia of Unknown Origin (PUO) remains one of the most challenging diagnostic puzzles in clinical medicine. First defined by Petersdorf and Beeson in 1961, PUO requires a systematic approach to uncover hidden infections, occult malignancies, or non-infectious inflammatory disorders. Understanding the changing definitions, classic etiologies, and diagnostic algorithms is essential for timely intervention and avoiding unnecessary empirical treatments.

Classical Definition and Evolution

The definition of PUO has evolved over time to account for modern outpatient care and advances in diagnostic testing:

• Petersdorf & Beeson Criteria (1961):

  • Illness duration $> 3$ weeks.

  • Fever $> 38.3^\circ\text{C}$ ($101^\circ\text{F}$) on several occasions.

  • Diagnosis uncertain after 1 week of inpatient investigation.

• Durack & Street Reclassification (1991):

  • Classic PUO: Temperature $> 38.3^\circ\text{C}$, duration $> 3$ weeks, with 3 outpatient visits or 3 days in the hospital without a cause.

  • Nosocomial PUO: Temperature $> 38.3^\circ\text{C}$ in a hospitalized patient receiving acute care, where infection was not present or incubating on admission.

  • Neutropenic PUO: Temperature $> 38.3^\circ\text{C}$ with an absolute neutrophil count (ANC) $< 500/\mu\text{L}$.

  • HIV-Associated PUO: Temperature $> 38.3^\circ\text{C}$ lasting $> 4$ weeks in outpatients or $> 3$ days in inpatients with confirmed HIV infection.

Etiological Categories of Classic PUO

The primary causes of Classic PUO fall into four major categories:

Category	Primary Etiologies	Diagnostic Clues
Infections (30–40%)	Extrapulmonary Tuberculosis, Infective Endocarditis, Intra-abdominal Abscesses, EBV, CMV.	Night sweats, heart murmurs, localized tenderness.
Malignancies (20–30%)	Hodgkin & Non-Hodgkin Lymphoma, Renal Cell Carcinoma, Leukemia, Hepatocellular Carcinoma.	B-symptoms, lymphadenopathy, organomegaly, weight loss.
Non-Infectious Inflammatory (15–20%)	Adult-onset Still Disease, Temporal Arteritis, Systemic Lupus Erythematosus (SLE), Polyarteritis Nodosa.	Joint pains, elevated ESR/CRP, rash, temporal headaches.
Miscellaneous (10–20%)	Drug fever, Subacute Thyroiditis, Pulmonary Embolism, Factitious Fever.	Drug history, thyroid tenderness, mismatch in pulse-fever.

Systematic Diagnostic Approach

1. Detailed History & Physical Examination: Re-evaluate exposure history, travel, drugs, and perform daily physical exams for subtle signs (e.g., Roth spots, lymph nodes, temporal artery tenderness).

2. First-Line Investigations: Complete Blood Count (CBC), ESR, CRP, Liver Function Tests, Blood Cultures ($\times 3$), Urinalysis, Chest X-ray, and Abdominal Ultrasonography.

3. Second-Line Investigations: Serologies (ANA, RF, HIV, Brucella), CT scan of Chest/Abdomen/Pelvis, and Echocardiography.

4. Advanced Imaging & Biopsy: $^{18}\text{F-FDG PET/CT}$ scan, Bone Marrow Aspirate & Biopsy, or targeted tissue biopsy.

Clinical Key Takeaway: Empiric antimicrobial therapy should generally be withheld in stable patients with classic PUO, as it may obscure diagnostic cultures and delay definitive identification of non-infectious causes.