Arthrocentesis of the Left Knee (Option B)
Arthrocentesis of the left knee is the most appropriate initial management for this patient to rule out septic arthritis, which is a medical emergency that can rapidly destroy the joint and lead to sepsis if not promptly diagnosed and treated.
Critical Clinical Context
This patient presents with a high-risk scenario for septic arthritis:
- Immunosuppression from triple therapy: Prednisone, methotrexate, and etanercept (a TNF inhibitor) all significantly impair immune function 1
- Acute monoarticular arthritis: New-onset swelling and pain in a single large joint over 3 days
- Systemic signs: Low-grade fever suggesting possible infection
- Established RA: The baseline synovitis in other joints (PIPs) confirms her underlying disease, making the acute knee presentation atypical 2
Why Arthrocentesis is Essential
Septic arthritis must be excluded immediately before any other diagnostic or therapeutic intervention. The synovial fluid analysis will:
- Differentiate infection from RA flare: Cell count, Gram stain, and culture are diagnostic 3, 4
- Guide immediate treatment: Septic arthritis requires urgent antibiotics and often surgical drainage to prevent irreversible cartilage destruction within 24-48 hours 5
- Assess crystal arthropathy: Gout or pseudogout can coexist with RA and present similarly 3
Patients on biologic DMARDs like etanercept have a significantly elevated infection risk, and septic arthritis can present with subtle findings in immunosuppressed patients 5.
Why Other Options Are Inappropriate
Bone scan (Option A): Has no role in acute monoarticular arthritis evaluation. It cannot distinguish infection from inflammation and would delay critical diagnosis 2.
Conventional radiography (Option C): Plain films show only late changes (joint space narrowing, erosions) and cannot diagnose acute septic arthritis or RA flare. While baseline radiographs are useful for RA monitoring, they are not the initial step in acute presentations 2, 6.
MRI (Option D): Though MRI can detect synovitis and early erosions, it cannot definitively exclude infection and would significantly delay diagnosis. Synovial fluid analysis remains the gold standard 2.
Clinical Algorithm After Arthrocentesis
If septic arthritis is confirmed (WBC >50,000/mm³, positive Gram stain/culture):
- Immediate IV antibiotics
- Orthopedic consultation for possible surgical drainage
- Hold all immunosuppressive medications temporarily 5
If inflammatory arthritis without infection (WBC 2,000-50,000/mm³, negative cultures):
- Consider RA flare: Intra-articular glucocorticoid injection provides rapid relief 2
- Assess disease activity and consider treatment intensification with rheumatology 2, 6
- Continue current DMARD regimen 2
If crystal arthropathy (crystals on polarized microscopy):
- Treat accordingly with NSAIDs or colchicine
- Intra-articular glucocorticoids are also effective 2
Critical Pitfall to Avoid
Never empirically treat as an RA flare without excluding infection first. Administering glucocorticoids (systemic or intra-articular) or increasing immunosuppression in undiagnosed septic arthritis can be catastrophic, leading to rapid joint destruction, sepsis, and death 5. The combination of immunosuppressive medications this patient is taking makes infection the most dangerous diagnosis that cannot be missed.