Diagnostic Approach for Fever of Unknown Origin
Definition and Classification
- Fever of unknown origin is defined as fever higher than 38.3°C persisting for at least 3 weeks, with no diagnosis despite 3 outpatient visits or in-patient days, according to the European Journal of Nuclear Medicine and Molecular Imaging 1
- The European Journal of Nuclear Medicine and Molecular Imaging classifies fever of unknown origin into four subcategories: classical, nosocomial, neutropenic, and HIV-related 1
- The etiology of fever of unknown origin includes infectious, inflammatory, malignant, and miscellaneous causes, with distribution varying by subcategory and geographical location, as reported by the European Journal of Nuclear Medicine and Molecular Imaging 1
Initial Diagnostic Approach
- The initial diagnostic approach for fever of unknown origin should include chest radiography, as recommended by Critical Care Medicine 2
- Blood cultures should be collected from different anatomical sites, with at least two sets of blood cultures (ideally 60 mL of blood total), according to Critical Care Medicine 2
- In patients with central venous catheters, simultaneous central and peripheral blood cultures should be obtained to calculate differential time to positivity, as recommended by Critical Care Medicine 2
- Inflammatory markers, including C-reactive protein and erythrocyte sedimentation rate, should be evaluated, as reported by the European Journal of Nuclear Medicine and Molecular Imaging 1
Advanced Diagnostic Workup
- If the initial evaluation is unrevealing, [18F]FDG PET/CT has a high diagnostic yield (56%) with sensitivity of 84-86%, according to the European Journal of Nuclear Medicine and Molecular Imaging 1
- PET/CT should ideally be performed within 3 days of starting oral glucocorticoid therapy, as recommended by the European Journal of Nuclear Medicine and Molecular Imaging 1
- For patients who have recently undergone thoracic, abdominal, or pelvic surgery, CT imaging of the operative area should be performed if an etiology is not readily identified, according to Critical Care Medicine 3
Treatment Approach
- Treatment should target the identified cause once diagnosis is established, as recommended by the World Journal of Emergency Surgery and other guidelines 4
- In severely neutropenic patients, broad-spectrum antibiotics with antipseudomonal activity should be initiated promptly, according to the World Journal of Emergency Surgery 4
- Antifungal therapy should be considered only in critically ill patients with persistent fever and new pulmonary infiltrates, as recommended by the World Journal of Emergency Surgery 4
Prognostic Considerations
- A negative [18F]FDG PET/CT can predict favorable prognosis through spontaneous remission of fever and potentially allows a watchful waiting approach, according to the European Journal of Nuclear Medicine and Molecular Imaging 1
- Early use of [18F]FDG PET/CT in the diagnostic workup has been demonstrated to be cost-effective, as reported by the European Journal of Nuclear Medicine and Molecular Imaging 1
Pitfalls to Avoid
- The Intensive Care Medicine journal recommends avoiding the diagnosis of "fever of unknown origin" prematurely, as it is a diagnosis of exclusion requiring thorough evaluation 5
- High-dose steroids should not be used without specific indication, as they increase the risk of hospital-acquired infection, hyperglycemia, gastrointestinal bleeding, and delirium, according to the Intensive Care Medicine journal 5
- Non-steroidal anti-inflammatory analgesics should be avoided, as they may impair renal and coagulation function and increase the risk of stress ulcer formation, as reported by the Intensive Care Medicine journal 5
Diagnostic Approach to Fever of Unknown Origin
Initial Evaluation
- The American College of Critical Care Medicine recommends beginning with chest radiography and at least two sets of blood cultures from different anatomical sites before initiating any antibiotics, including a comprehensive metabolic panel to identify hepatobiliary sources 6
- For patients with recent thoracic, abdominal, or pelvic surgery, the American College of Critical Care Medicine suggests performing CT of the operative area if fever occurs several days postoperatively and alternative cause not identified 6
- The American College of Critical Care Medicine also recommends performing formal diagnostic ultrasound of the abdomen for patients with abdominal symptoms, abnormal liver tests, or recent abdominal surgery 6
- Avoiding routine abdominal ultrasound or point-of-care ultrasound in patients without abdominal signs, symptoms, or liver function abnormalities is advised by the American College of Critical Care Medicine 6
Imaging Studies
- CT chest with IV contrast identifies pulmonary sources in 72% of surgical ICU patients with suspected infection, according to the American College of Radiology 7
- CT abdomen and pelvis with IV contrast has 81.82% positive predictive value for identifying septic foci, most commonly in abdomen and pelvis/genitourinary tract, as reported by the American College of Radiology 7
- These studies resulted in management changes in 45% of patients, including antimicrobial changes, surgery, and drainage catheter placement, according to the American College of Radiology 7
- The American College of Radiology advises against performing sinus CT routinely in prolonged febrile neutropenia without localizing symptoms, as abnormalities are common but non-discriminatory 8
Fever of Unknown Origin Management
Diagnostic Approach
- The Infectious Diseases Society of America recommends that for a patient presenting with fever of unknown origin (FUO), a structured diagnostic evaluation should be initiated, including at least two sets of blood cultures from different anatomical sites, without interval between them, before any antibiotic therapy, and the use of rapid molecular tests on blood should be done in conjunction with conventional blood cultures, never as a replacement 9
- The American College of Radiology suggests that a PET/CT scan with [18F]FDG can be used as a diagnostic tool with a high diagnostic yield of 56% and sensitivity of 84-86% if the initial evaluation is non-contributive 9
Therapeutic Management
- The Infectious Diseases Society of America recommends avoiding empirical antibiotic therapy in non-neutropenic patients with FUO, unless they are in a critical state, as most cases (up to 75%) resolve spontaneously without a definitive diagnosis 10
- The National Comprehensive Cancer Network recommends that for neutropenic patients, the risk stratification should guide the choice of antibiotic, with high-risk patients (neutropenia <100 cells/mm³ expected >7 days) initiating monotherapy with a β-lactam antibiotic such as piperacillin-tazobactam as first-line treatment 10
- The Infectious Diseases Society of America suggests that for patients with persistent fever under antibiotics, the fever alone rarely constitutes an indication to modify the antibiotic regimen, and empirical addition of vancomycin should not be done without microbiological documentation 10
Surveillance and Re-evaluation
- The Infectious Diseases Society of America recommends daily surveillance of all neutropenic patients under antibiotics to evaluate response, adverse effects, emergence of secondary infections, and development of resistant organisms, including a daily physical examination, review of systems for new symptoms, and cultures of suspect sites 10
- The Society suggests re-evaluation after 2-4 days of empirical antibiotic treatment, during which most modifications of the initial regimen will be made 10
Pitfalls to Avoid
- The American College of Radiology recommends not systematically removing central venous catheters in clinically stable patients with FUO, unless there is microbiological evidence of catheter-related infection 11
- The Infectious Diseases Society of America suggests avoiding non-directed antibiotic changes, and not switching from one empirical monotherapy to another or adding an aminoglycoside without clinical or microbiological data justifying an extension of the spectrum 10