Distinguishing Infectious, Malignant, and Autoimmune/Inflammatory Causes in Fever of Unknown Origin: A Clinical Review

Introduction and Definitions

Fever of unknown origin (FUO) remains one of the most challenging diagnostic dilemmas in internal medicine. The classic definition, established by Petersdorf and Beeson in 1961 and refined by Durack and Street in 1991, characterizes FUO as temperature ≥38.3°C (101°F) occurring on multiple occasions for ≥3 weeks without established diagnosis despite appropriate investigation 13. Modern criteria accommodate outpatient evaluation rather than mandating inpatient workup, reflecting contemporary diagnostic capabilities 13.

Four FUO subtypes are recognized: classic (community-acquired, immunocompetent), nosocomial (hospital-acquired after ≥24 hours), neutropenic (absolute neutrophil count <500/µL), and HIV-associated 13. This review focuses on classic FUO in adults, where the diagnostic challenge centers on distinguishing three major etiologic categories: infectious, malignant, and autoimmune/inflammatory causes.

Despite extensive diagnostic advances, up to 51% of FUO cases remain undiagnosed even after comprehensive evaluation, though prognosis is generally favorable with spontaneous resolution in most undiagnosed cases 13. Etiologic distribution varies by geography and healthcare setting: infectious causes predominate in developing countries, whereas non-infectious inflammatory disease is more common in developed nations 13.

Distinguishing Clinical Patterns and Constitutional Symptoms

Fever Pattern Taxonomy

Fever pattern analysis provides limited but occasionally helpful discriminatory information. A prospective study comparing 69 adult-onset Still's disease (AOSD) patients with 87 other FUO cases found that ≥3 daily fever peaks were negatively associated with AOSD diagnosis, suggesting that certain fever patterns may help narrow the differential 19. However, fever pattern alone lacks sufficient specificity to establish diagnosis and should be integrated with other clinical features.

Constitutional and "B Symptoms"

Constitutional symptoms—particularly the classic lymphoma-associated "B symptoms" (fever, drenching night sweats, unintentional weight loss >10% over 6 months)—raise concern for malignancy but are nonspecific. Weight loss trajectory, fatigue severity, and presence of cachexia provide additional context but require correlation with laboratory and imaging findings.

Localizing Signs and Symptoms

Careful repeated physical examination may reveal localizing clues:

• Cardiovascular: New cardiac murmurs, peripheral stigmata of endocarditis (splinter hemorrhages, Osler nodes, Janeway lesions), splenomegaly suggest infective endocarditis 20
• Musculoskeletal: Arthralgia (particularly large joints), salmon-pink evanescent rash, and sore throat form a discriminatory triad for AOSD 1924
• Head/neck: Temporal artery tenderness, jaw claudication, visual symptoms in patients >60 years suggest giant cell arteritis (GCA) 317
• Skin/mucosal: Evanescent rashes with fever spikes typify AOSD, whereas persistent erythematous rashes with subcutaneous nodules raise concern for panniculitis or subcutaneous infection 7
• Lymphadenopathy: Peripheral lymphadenopathy patterns help distinguish reactive processes from lymphoproliferative disorders and Kikuchi-Fujimoto disease 2

Epidemiology and Exposure History: Shifting Pre-Test Probability

Travel and Zoonotic Exposures

Geographic travel history identifies endemic infection risks: tuberculosis (global prevalence varies regionally), Q fever (Coxiella burnetii from livestock exposure) 27, brucellosis (unpasteurized dairy), typhoid (endemic regions), and histoplasmosis (endemic mycoses) 6. Occupational animal contact and dietary exposures provide critical epidemiologic clues often overlooked in standard history-taking 13.

Healthcare-Related Risk Factors

Prosthetic valves or intravascular devices increase endocarditis risk 20. Recent surgery, particularly thoracic, abdominal, or pelvic procedures, raises suspicion for deep abscess or anastomotic complications requiring targeted CT imaging 12. Immunosuppression from biologics or chemotherapy shifts the differential toward opportunistic infections and lymphoproliferative disorders 14.

High-Yield Laboratory and Biomarker Discriminators

Complete Blood Count and Differential

• Leukocytosis with neutrophilia: Bacterial infection, AOSD 19
• Cytopenias (pancytopenia): Hemophagocytic lymphohistiocytosis (HLH), bone marrow infiltration by lymphoma/leukemia, disseminated histoplasmosis 611
• Peripheral smear: Atypical lymphocytes, blasts, or schistocytes provide diagnostic clues

Inflammatory Markers and Specific Biomarkers

Acute-phase reactants: The 2023 SCCM/IDSA guideline meta-analysis demonstrated that procalcitonin (PCT) has superior diagnostic accuracy for bacterial sepsis (SROC 0.85, sensitivity 0.80, specificity 0.77) compared to C-reactive protein (CRP) (SROC 0.73, sensitivity 0.80, specificity 0.61) 12. PCT rises within 4 hours of bacterial exposure, whereas CRP rises over 12–24 hours 12. Importantly, PCT levels remain generally unaffected by non-infectious inflammatory processes, whereas CRP elevation is nonspecific 43. However, PCT should supplement rather than replace clinical judgment, particularly in low-to-intermediate probability scenarios 12.

Erythrocyte sedimentation rate (ESR) >100 mm/hr in elderly patients strongly suggests GCA or polymyalgia rheumatica, though this finding is nonspecific 325.

Ferritin: Serum ferritin >5× upper limit of normal (hyperferritinemia) is a discriminatory feature of AOSD in FUO workup 19. This biomarker helps distinguish AOSD from other inflammatory conditions, though HLH also produces marked hyperferritinemia and must be differentiated through bone marrow examination and additional criteria 611.

Lactate dehydrogenase (LDH): Elevated LDH appears as a recurring biomarker across malignant FUO causes—intravascular large B-cell lymphoma (IVLBCL), metastatic cancer, and HLH—and also in disseminated histoplasmosis 5610. When combined with other cytopenias and constitutional symptoms, LDH elevation should prompt bone marrow or targeted tissue biopsy.

Interleukin-18: Emerging evidence suggests IL-18 as a more specific biomarker for AOSD than ferritin, particularly in older patients, though availability remains limited 21.

Autoimmune Serologies: Interpretation and Pitfalls

Antinuclear antibodies (ANA), rheumatoid factor (RF), and antineutrophil cytoplasmic antibodies (ANCA) are useful when positive in appropriate clinical contexts. However, a critical case report documented false-positive melioidosis serology in a patient with AOSD, illustrating that autoimmune disorders can produce non-specific polyclonal B-cell expansion leading to false-positive infectious serologies 23. This underscores the principle that serological tests must be interpreted in clinical context rather than in isolation.

Blood Culture Strategy

At least two sets of blood cultures (ideally 60 mL total) drawn from different anatomical sites without time delay are recommended as best practice 12. For patients with central venous catheters, simultaneous peripheral and catheter-drawn cultures allow differential time-to-positivity calculation to diagnose catheter-related bloodstream infection 12. Culture-negative endocarditis necessitates serological testing for fastidious organisms (Coxiella burnetii, Bartonella, Legionella) and molecular methods 2027.

Imaging Strategies That Separate Etiologic Categories

Conventional Cross-Sectional Imaging

Chest radiography is recommended as best practice for all critically ill patients with new fever (ICU) given pneumonia prevalence 12. CT chest/abdomen/pelvis is indicated for patients with recent surgery when initial workup is unrevealing 12 and serves as a foundational modality for identifying abscesses, lymphadenopathy, and occult malignancy.

Echocardiography

Transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE) are central to infective endocarditis diagnosis via modified Duke criteria 20. TEE offers superior sensitivity for detecting small vegetations, paravalvular abscesses, and prosthetic valve involvement compared to TTE, though specific sensitivity values were not provided in retrieved literature 20.

Advanced Imaging: FDG-PET/CT

A 2025 review established FDG-PET/CT as a critical diagnostic tool in FUO evaluation after conventional investigations have been exhausted 1. As a whole-body imaging technique with high sensitivity for increased metabolism, FDG-PET/CT identifies:

• Infectious sources: Metastatic infections, endocarditis, clinically silent abscesses, osteomyelitis 1
• Malignancies: Occult lymphoma (including IVLBCL detected via lung biopsy after PET-avid lesions identified) 10, and incidental malignancies 29
• Inflammatory conditions: Large-vessel vasculitis, HLH (may show diffuse marrow/splenic uptake and help guide biopsy) 111

The 2023 SCCM/IDSA guideline suggests FDG-PET/CT for critically ill patients when other diagnostic tests have failed, provided transport risk is acceptable 12. A systematic review and Delphi consensus demonstrated that PET/CT diagnostic yield varies by FUO phenotype, with higher utility in inflammatory and malignant versus infectious presentations 18.

Critical diagnostic pitfall: A 2025 case demonstrated extensive subcutaneous infection producing a "snow leopard sign" on FDG-PET/CT, initially misinterpreted as subcutaneous panniculitis-like T-cell lymphoma 7. Tissue biopsy confirmed infection, illustrating that PET-avid lesions require histopathologic confirmation before assuming malignancy.

Ultrasound and MRI for Large-Vessel Vasculitis

Temporal artery ultrasound demonstrating the "halo sign" achieves 77% sensitivity and 96% specificity for GCA diagnosis compared to clinical diagnosis, with sensitivity increasing to 87% when high-quality equipment (≥12 MHz probes) is used 17. MRI of cranial arteries shows 73% sensitivity and 88% specificity versus clinical diagnosis 17. A multimodal diagnostic pathway combining ultrasonography with temporal artery biopsy or FDG-PET/CT achieved 95.7% sensitivity and 98.0% negative predictive value for GCA, substantially outperforming superficial temporal artery imaging alone (62.3% sensitivity, 84.7% NPV) 3.

Tissue Biopsy: Indications and Yield

When Conventional Workup Fails

Tissue diagnosis remains essential when imaging findings are nonspecific or misleading. Multiple case reports illustrate this principle:

• Bone marrow biopsy: Confirmed metastatic bone cancer after enhanced CT and FDG-PET/CT were inconclusive, requiring repeated bone marrow pathology 5; diagnosed HLH secondary to peripheral T-cell lymphoma after highly FDG-avid lung lesion biopsy 11; revealed intravascular large B-cell lymphoma when renal biopsy identified atypical B-cells in glomerular capillaries 9
• Lymph node biopsy: Surgical excisional biopsy diagnosed Kikuchi-Fujimoto disease in a patient with prolonged FUO and cervical lymphadenopathy 2
• Temporal artery biopsy: Despite high specificity, TAB lacks sensitivity with false-negative results in up to 61% of GCA patients due to skip lesions and extracranial vessel predominance in 40% of cases 17
• Skin/subcutaneous tissue biopsy: Differentiated subcutaneous infection from malignancy when PET/CT findings were ambiguous 7

Category-Specific Red Flags and Prototypical Diagnoses

Practical Diagnostic Algorithm and Common Pitfalls

Stepwise Workup Approach

1. Initial evaluation: Comprehensive history (travel, exposures, medications), repeated physical examination, baseline laboratory tests (CBC with differential, CMP, ESR/CRP, ferritin, LDH, urinalysis), ≥3 sets of blood cultures, chest radiograph 13

2. Targeted testing based on clinical suspicion: Autoimmune serologies if multisystem features present; echocardiography if endocarditis suspected; tuberculin skin test or IGRA if TB risk; age-appropriate malignancy screening 13

3. Advanced imaging: CT chest/abdomen/pelvis for occult infection or malignancy; temporal artery ultrasound or MRI if GCA suspected in elderly patients 317

4. FDG-PET/CT: Reserved for cases where conventional workup unrevealing, to identify occult inflammatory foci, large-vessel vasculitis, or malignancy and guide biopsy site selection 11218

5. Tissue biopsy: Directed by imaging findings or clinical suspicion—lymph node, bone marrow, temporal artery, skin, or organ-specific biopsy 25911

Common Diagnostic Pitfalls

• Premature empiric therapy: Empiric antibiotics are not indicated in immunocompetent classic FUO except for deteriorating patients or specific high-suspicion diagnoses (culture-negative endocarditis, cryptic tuberculosis, temporal arteritis with vision threat) 13. Empiric glucocorticoids obscure autoimmune/inflammatory diagnoses and should be avoided unless strong clinical indication exists 13

• Anchoring bias: Initial diagnostic impressions (e.g., presumed infection) may delay recognition of malignancy or autoimmune disease. Serial reassessment and willingness to reconsider the differential are essential.

• Over-reliance on imaging without tissue confirmation: As illustrated by the subcutaneous infection mimicking lymphoma case 7, PET-avid lesions require biopsy confirmation before attributing findings to malignancy.

• Underappreciation of drug fever: Drug-induced fever represents an important non-infectious mimic and should be considered when temporal relationship to medication administration exists 1326.

• Missed culture-negative endocarditis: Fastidious organism serology (Coxiella, Bartonella) must be pursued when clinical suspicion for endocarditis is high despite negative blood cultures 2027.

Key Take-Home Points

1. FUO diagnosis requires systematic integration of clinical phenotyping, epidemiologic risk assessment, laboratory biomarkers, and stepwise imaging rather than isolated test interpretation.

2. Procalcitonin outperforms CRP for bacterial infection discrimination (SROC 0.85 vs 0.73), though low PCT does not exclude infection in all contexts 1243.

3. Hyperferritinemia (>5× ULN), arthralgia, sore throat, and neutrophilia form a discriminatory constellation for AOSD 19; elevated LDH suggests malignancy or HLH 5610.

4. FDG-PET/CT is most useful after conventional workup is unrevealing and helps identify inflammatory/malignant causes and direct tissue biopsy, but PET-avid lesions require histopathologic confirmation 1718.

5. Temporal artery ultrasound (77% sensitivity, 96% specificity) and multimodal imaging pathways substantially improve GCA diagnostic yield over biopsy alone 317.

6. Empiric antimicrobial or immunosuppressive therapy should be avoided in stable immunocompetent patients to preserve diagnostic opportunities, with exceptions for specific high-risk scenarios 13.

7. Despite comprehensive evaluation, up to 51% of FUO cases remain undiagnosed, though most have favorable prognosis with spontaneous resolution 13.