In a patient with cyclic fevers, poorly controlled joint pain and swelling, and no nausea, vomiting, diarrhea, dyspnea, or chest pain, what are the differential diagnoses and recommended work‑up?

Differential Diagnosis and Work-Up for Cyclic Fevers with Joint Pain and Swelling

Your patient most likely has an autoinflammatory disorder, with Adult-Onset Still's Disease (AOSD) being the primary consideration, followed by familial Mediterranean fever (FMF) and other periodic fever syndromes. The cyclic nature of fevers combined with poorly controlled joint symptoms strongly suggests an autoinflammatory process rather than infection or malignancy 1, 2.

Key Differential Diagnoses to Consider

Primary Considerations:

• Adult-Onset Still's Disease (AOSD): Characterized by quotidian (daily spiking) fevers >39°C, evanescent salmon-pink rash, arthralgia/arthritis, sore throat, lymphadenopathy, and markedly elevated ferritin (often >5x normal with low glycosylated fraction <20%) 1

• Familial Mediterranean Fever (FMF): Recurrent 1-3 day fever episodes with serositis, arthritis (especially knee, ankle, hip), and erysipelas-like erythema; consider especially in Mediterranean, Middle Eastern, or Armenian ancestry 1, 3

• Other Autoinflammatory Syndromes: TRAPS (longer fever episodes averaging 21 days with ocular involvement), PFAPA syndrome, or hyperimmunoglobulin D syndrome 1, 2, 4

Critical Alternative Diagnoses to Exclude:

• Infections: Bacterial endocarditis, tuberculosis, viral infections (EBV, CMV, HIV, hepatitis), or occult abscess 1, 5, 6

• Malignancies: Lymphoma (Hodgkin's and non-Hodgkin's), leukemia, or solid tumors with paraneoplastic manifestations 1, 5

• Rheumatoid Arthritis: Especially if symmetrical small joint involvement, morning stiffness >30 minutes, positive RF or anti-CCP antibodies 1

• Systemic Lupus Erythematosus: Consider if ANA positive with multi-system involvement 1

• Reactive Arthritis/Spondyloarthropathies: Particularly if axial or entheseal involvement, HLA-B27 positive 1

Comprehensive Work-Up Algorithm

Initial History Focus 1:

• Fever pattern: Exact timing, duration, and periodicity of fever spikes (quotidian vs. irregular)
• Geographic/ethnic background: Mediterranean, Middle Eastern, Armenian ancestry suggests FMF 1
• Joint pattern: Number of joints, symmetry, small vs. large joints, morning stiffness duration
• Associated symptoms: Rash (timing with fever), sore throat, lymphadenopathy, serositis symptoms
• Family history: Recurrent fevers, autoinflammatory diseases, autoimmune conditions
• Functional status: HAQ score or similar disability assessment 1

Essential Baseline Laboratory Tests 1:

Tier 1 - Mandatory Initial Tests:

• Complete blood count with differential: Look for leukocytosis (>10,000 with >80% granulocytes suggests AOSD) 1
• ESR and CRP: Both should be performed; elevated levels support inflammatory process 1
• Comprehensive metabolic panel: Including liver function tests, renal function, glucose, urate 1
• Ferritin level: Markedly elevated (>5x normal) with low glycosylated fraction (<20%) highly specific for AOSD 1
• Rheumatoid factor (RF) and anti-CCP antibodies: Essential for RA evaluation 1
• Antinuclear antibodies (ANA): Screen for SLE and other connective tissue diseases 1

Tier 2 - Critical Additional Tests:

• Blood cultures (aerobic and anaerobic): Minimum 2 sets if fever present to exclude endocarditis 1, 5
• Urinalysis and urine culture: Rule out occult urinary infection 1
• Lactate dehydrogenase (LDH) and creatine kinase: Evaluate for tissue damage or malignancy 5

Tier 3 - Specialized Serologic Testing:

• IL-18 and S100 proteins (calprotectin, S100A12): If available, highly sensitive and specific for Still's disease (IL-18 >1157 pg/mL suggests AOSD) 1
• HLA-B27 typing: If axial symptoms or suspicion of spondyloarthropathy 1
• Region-specific serologies: HIV, hepatitis panel, EBV, CMV, tuberculosis testing based on risk factors 5
• Anti-dsDNA and extractable nuclear antigens: If ANA positive 1
• Immunoglobulins including IgD: Elevated IgD suggests hyperimmunoglobulin D syndrome 1

Imaging Studies 1:

• Chest radiograph: Mandatory to exclude infection, malignancy, or serositis 1
• Hand and feet radiographs: Assess for erosions, periarticular osteopenia, or characteristic patterns 1
• Abdominal/pelvic CT or ultrasound: If abdominal symptoms present or to exclude occult malignancy 5
• 18F-FDG PET/CT: Consider if diagnosis remains elusive after initial workup; can guide tissue biopsy 5

Additional Diagnostic Procedures When Indicated:

• Arthrocentesis with synovial fluid analysis: Perform if monoarthritis or oligoarthritis; send for cell count, differential, culture (aerobic/anaerobic), and crystal analysis 1
• Bone marrow biopsy: If cytopenias, elevated LDH, or concern for hematologic malignancy 5
• Tissue biopsy: Lymph node, skin, or other affected tissue if lymphadenopathy or rash present 5

Critical Diagnostic Pitfalls to Avoid

Do not start empiric antibiotics or corticosteroids before obtaining cultures and completing initial diagnostic workup, as this significantly reduces diagnostic yield and may mask serious infections or malignancies 1, 5. If the patient is medically stable, withhold antimicrobials for at least 2 weeks prior to culture collection 1.

AOSD is a diagnosis of exclusion—you must systematically rule out infections, malignancies, and other immune-mediated diseases before confirming this diagnosis 1. The presence of elevated ferritin alone is insufficient; the glycosylated fraction must be low (<20%) 1.

Consider genetic testing for MEFV gene if FMF is suspected based on ethnicity and clinical pattern, as this can confirm diagnosis and guide colchicine therapy 1, 3.

Monitor for macrophage activation syndrome (MAS), a life-threatening complication of Still's disease, characterized by sudden worsening with cytopenias, elevated ferritin, elevated triglycerides, and decreased fibrinogen 1.