Fever & Inflammation of Unknown Origin¹⁸F-FDG · ⁶⁷Ga

Quick Answer

Fever of unknown origin (FUO) — recurrent fever without a diagnosis after an appropriate initial workup — and its afebrile cousin inflammation of unknown origin (IUO) are classic whole-body search problems, and ¹⁸F-FDG-PET/CT has become the highest-yield imaging test for them. FDG is a nonspecific marker of increased glucose metabolism in infection, inflammation, and malignancy — which is a weakness for characterizing a known lesion but a strength here, because the three causes of FUO (infection, non-infectious inflammatory disease, malignancy) are exactly the FDG-avid categories. A single whole-body scan surveys the entire body for a metabolic focus to direct further, targeted testing (biopsy, culture, dedicated imaging). It has largely replaced ⁶⁷Ga (which required delayed multi-day imaging with inferior resolution). A negative FDG-PET is also useful — it makes occult focal disease less likely and is associated with a higher rate of spontaneous resolution.

Why FDG fits the problem

FDG accumulates wherever glucose metabolism is high — activated inflammatory and immune cells, and tumor. For a specific lesion this nonspecificity is a limitation, but for FUO it is ideal: the differential (infection, inflammatory/autoimmune disease such as large-vessel vasculitis or adult-onset Still's, and malignancy such as lymphoma) spans precisely the FDG-avid processes, so one whole-body scan screens all three categories at once and points to the next targeted test.

Yield and how to use it

FDG-PET/CT changes management in a substantial share of FUO/IUO workups by revealing an occult focus — vasculitis, endocarditis/endovascular infection, abscess, osteomyelitis/spondylodiscitis, or lymphoma — that then guides biopsy or culture. Practical points:

Reading with care

The same nonspecificity that helps also demands discipline: physiologic and benign FDG uptake (bowel, muscle, brown fat, marrow after G-CSF) and post-procedural inflammation must not be over-called, and a focus is a lead, not a diagnosis — it needs histologic or microbiologic confirmation. Patient preparation (fasting/glucose control) matters, and recent antibiotics or steroids can suppress inflammatory uptake and lower yield. The physician's job is to convert an avid focus into a specific next step.

High-Yield Pearls

• FDG-PET/CT is the highest-yield imaging test for FUO/IUO — one whole-body scan screens infection, inflammation, and malignancy.
• FDG's nonspecificity is a strength here because it matches the FUO differential.
• It localizes a focus to direct biopsy/culture — it rarely makes the final diagnosis alone.
• A negative scan is informative (less occult focal disease; higher spontaneous-resolution rate).
• It has largely replaced ⁶⁷Ga (faster, sharper, higher yield).

Common Pitfalls

• Over-calling physiologic/benign uptake (bowel, brown fat, marrow) as the culprit.
• Expecting the scan to make a final diagnosis rather than direct targeted testing.
• Reduced yield after recent antibiotics/steroids suppressing inflammatory uptake.

Related Pages

• Overview: Infection & inflammation imaging; entities: Large-vessel vasculitis, Endocarditis/device infection, Spondylodiscitis; pitfalls: Physiologic FDG mimics.

Differential Diagnosis

• Large-vessel vasculitis (giant-cell arteritis, Takayasu, PMR) — smooth, linear uptake along the aorta and great-vessel walls.
• Endovascular / cardiac infection — focal uptake at native or prosthetic valves, vascular grafts, or CIED leads (needs dedicated cardiac preparation).
• Occult malignancy — lymphoma (often intensely avid) and other solid or hematologic tumors presenting as fever.
• Focal pyogenic infection — abscess, spondylodiscitis/osteomyelitis, or an infected collection to localize for biopsy/culture.
• Non-infectious inflammatory disease — adult-onset Still's, sarcoidosis, IgG4-related disease, inflammatory bowel disease.

Reporting Checklist

• State patient preparation and glucose, and recent antibiotics/steroids, which affect sensitivity.
• Report each focus: site, intensity, and whether it appears physiologic/benign or pathologic.
• Specifically address vascular-wall, valvular/perigraft, nodal, and splenic/marrow uptake relevant to the FUO categories.
• Recommend the specific next step (biopsy, culture, dedicated imaging) for the leading focus.
• For a negative study, state the reduced likelihood of occult focal disease and the higher spontaneous-resolution rate.

Self-Check

Key References

• SNMMI/EANM guidelines on FDG-PET/CT for fever and inflammation of unknown origin.
• Cohort/meta-analytic data on FDG-PET/CT diagnostic yield and the prognostic value of a negative scan in FUO.