Comparison of FDG-PET/CT and CECT-CAP as Upfront Imaging Modality for the Diagnosis of Pyrexia of Unknown Origin

Pyrexia of unknown origin (PUO) remains one of the most challenging diagnostic problems in modern medicine. Despite advances in laboratory diagnostics, microbiological techniques, imaging modalities, and molecular testing, a significant proportion of patients continue to undergo multiple, prolonged, expensive, and often invasive evaluations before a definitive diagnosis is established. PUO is classically defined as fever ≥38.3°C documented on ≥2 occasions AND duration ≥3 weeks or inpatient fever ≥1 week without diagnosis despite routine evaluation.

The etiological spectrum of PUO is broad and heterogeneous. Causes are traditionally grouped into infections, malignancies, non-infectious inflammatory diseases, miscellaneous causes, and undiagnosed cases. Despite systematic evaluation, a definitive diagnosis is not achieved in a substantial proportion of patients, often ranging from 30-50% even in tertiary care centers. This persistent diagnostic uncertainty highlights the need for effective diagnostic strategies that can identify occult disease early and guide targeted investigations.

Imaging plays a pivotal role once baseline routine investigations fail to establish a diagnosis. Conventional contrast-enhanced computed tomography (CECT) of the chest, abdomen, and pelvis is frequently used due to its wide availability and ability to detect structural abnormalities such as abscesses, masses, and lymphadenopathy. However, CECT relies on anatomical changes and may fail to identify early inflammatory or infiltrative disease that lacks overt structural distortion.

In contrast, nuclear medicine modalities enable the identification of pathological processes at the cellular and molecular level. 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) exploits abnormal glucose metabolism in inflammatory, infectious, and malignant tissues. When combined with PET/CT, it enables precise anatomical localization of metabolically active lesions. Numerous studies have shown that FDG PET/CT is valuable in the evaluation of PUO, especially when standard imaging modalities fail to yield a diagnosis.

In a retrospective study, FDG-PET/CT outperformed CECT-CAP for definite diagnosis of PUO with markedly higher localisation (92.2 vs. 51.5 %; p = 0.003). In a comparative multicentre prospective study FDG PET/CT demonstrated significantly superior diagnostic orientation (28.2% vs. 7.8%, p \< 0.001) and diagnostic contribution (19.4% vs. 5.8%, p \< 0.001) compared with CECT-CAP, and was associated with a markedly shorter time to definitive diagnosis (3.8 vs. 17.6 months, p = 0.02). However in these studies same patient underwent both FDG-PET/CT and CECT-CAP. In a systematic review, meta-analysis FDG PET/CT had pooled diagnostic yield of 56% and diagnostic yield beyond conventional CT at 32%. However another systematic review/meta-analysis similarly reported a higher contributory effect of FDG-PET/CT compared to Total Body CT (75.4% vs 68%,P = .39) for final diagnosis of PUO although it was not statistically significant. However both studies included data from retrospective studies.

Current clinical practice varies with respect to the timing and sequencing of FDG PET/CT and Conventional CECT in PUO evaluation. FDG PET/CT is often used as a second-line investigation after CECT-CAP or when CECT-CAP is inconclusive; however, growing evidence suggests that early use of FDG PET/CT may offer superior diagnostic yield and reduce diagnostic delay. However clear evidence from randomized controlled trials comparing FDG PET/CT and CECT-CAP as upfront investigations is lacking. This uncertainty forms the basis of clinical equipoise and provides the rationale for the present study. In this study investigators will evaluate and compare the diagnostic localization and diagnostic yield of FDG PET/CT over CECT-CAP for etiological diagnosis of PUO.