What is the appropriate management for a patient with rheumatoid arthritis presenting with acute left knee pain, fever, and recent steroid injection?

Immediate Management: Septic Arthritis Until Proven Otherwise

This patient requires urgent arthrocentesis of the left knee with synovial fluid analysis (cell count, Gram stain, culture) and empiric intravenous antibiotics should be initiated immediately if septic arthritis is confirmed or strongly suspected. 1, 2

Critical Clinical Context

This presentation is highly concerning for septic arthritis, not a rheumatoid arthritis flare, based on:

• Fever (38°C) with acute monoarticular knee pain in an immunosuppressed patient 2
• Recent intra-articular steroid injection (1 month ago) - a known risk factor for introducing infection 1
• Methotrexate therapy - significant immunosuppression increasing infection risk 2
• Diabetes mellitus - independent risk factor for septic arthritis 2
• Tachycardia (pulse 105) suggesting systemic inflammatory response 2

The EULAR guidelines explicitly state: "Intra-articular steroids should not be administered unless an appropriate diagnosis has been made and contraindications have been ruled out" - and conversely, when a patient presents with acute joint swelling after recent injection, infection must be excluded first 1

Immediate Diagnostic Workup

Synovial fluid analysis is mandatory and should include: 2

• Cell count with differential (septic arthritis typically >50,000 WBC/μL with >90% neutrophils, but lower counts don't exclude infection in immunosuppressed patients)
• Gram stain and culture (positive in 50-70% of cases; Staphylococcus aureus is most common pathogen)
• Crystal analysis (to exclude gout/pseudogout, though these can coexist with infection)

Serum inflammatory markers: 2

• ESR and CRP (helpful but nonspecific - elevated in both RA flare and infection)
• Blood cultures (positive in 25-50% of septic arthritis cases)

Common pitfall: In rheumatoid arthritis patients, septic arthritis can masquerade as a disease flare with polyarticular symptoms, and initial synovial fluid analysis may not show classic findings - maintain high suspicion even if presentation seems atypical 3

Empiric Antibiotic Therapy

If septic arthritis is confirmed or clinical suspicion is high, initiate antibiotics immediately after obtaining synovial fluid: 2

• Vancomycin 15-20 mg/kg IV every 8-12 hours (covers MRSA, most common in healthcare-associated and post-injection cases)
• Plus ceftriaxone 1-2g IV daily or cefepime 2g IV every 8 hours (covers gram-negative organisms)

Oral antibiotics are non-inferior to IV therapy for most cases and can be transitioned once culture results guide therapy, with total duration of 2-6 weeks depending on organism and clinical response 2

Surgical Consultation

Orthopedic surgery consultation for joint drainage is indicated: 2

• Arthroscopic or open drainage may be required if:
  • Large effusion with loculations
  • Inadequate response to antibiotics within 48-72 hours
  • Prosthetic joint (though not mentioned in this case)

Serial arthrocentesis (daily or every other day) is an alternative to surgical drainage for native joints with good initial response 2

Risk Stratification for Infection Post-Steroid Injection

The 2023 ACR/AAHKS guidelines note that intra-articular glucocorticoid injection within 3 months of surgery increases infection risk, and the 2021 EULAR recommendations state injections should be performed at least 3 months prior to joint replacement 1

This patient's injection 1 month ago falls within the high-risk window for post-injection septic arthritis, particularly given her immunosuppression from methotrexate and diabetes 1

Management of Immunosuppression

Hold methotrexate immediately until infection is excluded or adequately treated 1

Continue current glucocorticoid dose (if patient is on chronic steroids for RA) - do not abruptly discontinue, but do not add stress-dose steroids 1

Differential Diagnosis Considerations

While septic arthritis is the primary concern, also consider:

• Crystal arthropathy (gout/pseudogout) - can be triggered by steroid injection, but fever and systemic symptoms less prominent 1
• RA flare - less likely given monoarticular presentation, fever, and recent steroid injection that should have suppressed inflammation 3
• Post-injection inflammatory reaction - typically occurs within 24-48 hours of injection, not 1 month later 1

The presence of fever, tachycardia, and acute presentation 1 month post-injection makes septic arthritis the most likely diagnosis requiring immediate intervention 3, 2