FDG PET/CT in Pyrexia of Unknown Origin: A Review

Abstract

Pyrexia of unknown origin (PUO) was initially defined by Petersdorf and Beeson in 1961 as "a condition in which a fever of 38.3 °C or higher is observed several times for more than 3 weeks with an unknown cause after in-hospital investigation for 1 week or longer"¹. However with improvements in diagnostic modalities and better understanding of various diseases PUO has been recently redefined as (1) fever ≥ 38.3 °C (≥ 101 °F) on at least two occasions, (2) illness duration of ≥ 3 weeks, (3) no known immune-compromised state, (4) uncertain diagnosis even after thorough history-taking, physical examination, and the following obligatory investigations: determination of erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) level, platelet count, leukocyte count, hemoglobin (Hb), electrolytes, creatinine, total protein, alkaline phosphatase, alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, creatine kinase, ferritin, antinuclear antibodies, rheumatoid factor, protein electrophoresis, urinalysis, blood cultures, urine culture, chest X-ray, abdominal ultrasonography, and tuberculin skin test or interferon-γ release assay². If the diagnosis remains uncertain after these investigations, different protocols have been set up with an ever-increasing array of invasive and noninvasive diagnostic modalities. 18F-fluorodeoxyglucose positron emission tomography (FDG PET) is one such modality. Whereas conventional imaging modalities depend more on anatomical changes caused by disease processes, PET combines anatomical and functional assessment, thus increasing sensitivity and specificity.