What is the diagnosis and treatment approach for septic arthritis?

Diagnosis and Management of Septic Arthritis

Definitive diagnosis of septic arthritis requires joint aspiration with synovial fluid analysis, which should be performed immediately when clinical suspicion exists to prevent permanent joint damage and significant morbidity. 1

Clinical Presentation and Risk Assessment

• Key clinical features:

  • Acute onset of monoarticular joint pain, erythema, heat, and limited mobility
  • Constitutional symptoms (fever, chills) may be present but are not sensitive indicators
  • Risk factors: age >80 years, diabetes mellitus, rheumatoid arthritis, recent joint surgery, prosthetic joints, skin infection, and immunosuppression 2

• Kocher criteria for pediatric hip septic arthritis:

  • Fever >101.3°F
  • ESR ≥40 mm/hour
  • WBC count ≥12,000 cells/mm³
  • Inability to bear weight
  • Nearly 100% likelihood of septic arthritis if all criteria are met 1

Diagnostic Algorithm

1. Initial Imaging

• Plain radiographs should be obtained first to identify soft tissue swelling, joint effusion, gas, and exclude fractures or tumors 1

2. Advanced Imaging (if clinically indicated)

• Ultrasound if radiographs show effusion or are normal but clinical suspicion remains high

  • Highly sensitive for detecting joint effusions
  • Guides joint aspiration
  • Particularly valuable in children (no radiation, no sedation needed) 1

• MRI with contrast indicated when:

  • Joint aspiration is inconclusive or technically difficult
  • Need to evaluate infection extent beyond joint space
  • Suspicion of concurrent osteomyelitis (common in pediatric patients)
  • Findings include: joint effusion, synovial enhancement, bone marrow edema 1, 3

3. Definitive Diagnosis: Joint Aspiration and Synovial Fluid Analysis

• Send synovial fluid for:

  • Cell count and differential
  • Gram stain
  • Culture and sensitivity
  • Crystal analysis (to rule out concurrent crystal arthropathy) 1

• Diagnostic criteria:

  • Synovial fluid WBC count typically >50,000/mm³ in native joints
  • For prosthetic joints, lower threshold applies: >1,100 WBC/mm³ with neutrophil differential >64% 4, 1
  • Positive culture is definitive but should not delay treatment 2

Treatment Approach

1. Immediate Management

• Obtain synovial fluid before initiating antibiotics when possible
• Start empiric antibiotics after fluid collection if clinical suspicion remains high 1

2. Antibiotic Selection

• Initial empiric therapy based on Gram stain:
  • Gram-positive cocci: Vancomycin
  • Gram-negative cocci: Ceftriaxone
  • Gram-negative rods: Ceftazidime
  • If Gram stain negative but high clinical suspicion: Vancomycin plus ceftazidime or an aminoglycoside 4

3. Joint Drainage

• Evacuation of purulent material is essential through:
  • Repeated arthrocentesis
  • Arthroscopic drainage
  • Open surgical drainage (arthrotomy) for extensive infection 1, 5

4. Duration of Therapy

• Continue appropriate antibiotics for 2-6 weeks
• Oral antibiotics can be used in most cases as they are not inferior to IV therapy 1, 2
• Monitor inflammatory markers (CRP, ESR) to assess treatment response

Special Considerations

• Prosthetic joint infections require specialized management:

  • Longer antibiotic courses
  • Often hardware removal
  • Lower threshold for diagnosis (WBC >1,100/mm³) 1, 4

• Concurrent infections:

  • Osteomyelitis should be suspected, especially in pediatric patients
  • MRI with contrast is highly sensitive for detecting concurrent bone infection 1, 3

• Atypical pathogens to consider based on history:

  • Neisseria gonorrhoeae
  • Borrelia burgdorferi
  • Fungal infections 2

• Coexisting conditions:

  • Crystal arthropathy can coexist with septic arthritis
  • End-stage renal disease patients have higher incidence (514.8 per 100,000 persons per year) 6

Pitfalls to Avoid

• Delaying joint aspiration when septic arthritis is suspected
• Starting antibiotics before obtaining synovial fluid (when possible)
• Overlooking concurrent osteomyelitis, especially in children
• Missing coexistent crystal arthropathy
• Inadequate joint drainage or insufficient antibiotic duration