In a patient with sepsis of unknown source, which FDG‑PET/CT (fluorodeoxyglucose positron emission tomography/computed tomography) is most helpful for detecting occult septic foci?

FDG-PET/CT for Detecting Septic Foci

FDG-PET/CT (skull base to mid-thigh) is most helpful for detecting occult septic foci in patients with sepsis of unknown origin AFTER initial conventional imaging (CT chest/abdomen/pelvis with IV contrast) has failed to identify a source. 1

Diagnostic Algorithm

First-Line Imaging

• CT chest with IV contrast for suspected pulmonary source (detects 38.6% of septic foci, PPV 81.8%) 1
• CT abdomen/pelvis with IV contrast for suspected abdominal/pelvic source (detects 22% abdominal, 20.5% pelvic/GU foci) 1

When to Use FDG-PET/CT

FDG-PET/CT should only be ordered after CT has been unrevealing - it is NOT appropriate as initial imaging after chest X-ray alone 1. The ACR guidelines explicitly state that PET/CT lacks the anatomic localization of diagnostic CT and should be reserved for cases where source localization with CT has failed 1.

Diagnostic Performance of FDG-PET/CT

In Sepsis of Unknown Origin

• Sensitivity: 90.9-94% 1
• Specificity: 77-87.5% 1
• Positive findings: 66-78% of scans 1
• NPV: 77.8-100% (excellent for ruling out occult foci) 1

In Bacteremia of Unknown Origin

• Detection rate: 56.4% of infection sites 1
• First modality to identify source: 41.1% of cases 1
• High clinical impact: 47.3% of cases 1

Most Common Foci Detected

1. Musculoskeletal (38%) - including spondylodiscitis, septic arthritis 1
2. Chest/pulmonary (25%) - pneumonia, empyema 1
3. Abdominal/pelvic sources 1

Clinical Impact

Treatment modifications occur in 25-52% of patients based on FDG-PET/CT findings 1:

• Surgical interventions (abscess drainage, removal of infected material)
• Placement of drainage catheters
• Changes in antimicrobial therapy (initiation, modification, or prolongation)

The 2025 EANM/SNMMI guidelines report a diagnostic yield of 56% with diagnostic accuracy of 76% and clinical helpfulness in 61% of cases for fever/inflammation of unknown origin 2.

Factors Predicting Positive PET/CT

Higher likelihood of positive findings:

• Normal serum AST levels
• Elevated alkaline phosphatase 3
• Absence of liver cirrhosis 1
• Longer pre-scan ICU stay 4

Lower diagnostic yield:

• Diabetes mellitus 4
• Partial-body vs. total-body imaging 4

Critical Caveats

Limitations

• False positive rate: 17% in some series 1
• Suboptimal myocardial suppression can occur with male sex and unfractionated heparin use 4
• Scan quality affected by higher BMI (improves quality) and lower creatinine (improves quality) 4

Safety Profile

No adverse events during transport or scanning reported in most studies 5, 6, making it safe even for critically ill ICU patients.

Cost-Effectiveness

Early use of FDG-PET/CT in the diagnostic workup has demonstrated cost-effectiveness, particularly in high-risk populations 2. However, this applies to strategic patient selection, not routine use.

Practical Recommendation

Use FDG-PET/CT when:

1. Patient has culture-proven bacteremia or clinical sepsis
2. Initial CT chest/abdomen/pelvis with contrast is negative or equivocal
3. Source remains unknown after 72 hours of conventional workup
4. Patient is stable enough for transport (though adverse events are rare)

Order total-body imaging (skull base to mid-thigh) rather than partial-body to maximize diagnostic yield 4.