What are the diagnostic criteria and treatment options for septic arthritis?

Diagnostic Criteria for Septic Arthritis

Septic arthritis is diagnosed definitively by joint aspiration with synovial fluid white blood cell count ≥50,000 cells/mm³, positive culture, and clinical presentation of acute monoarticular joint pain, swelling, and fever, though this classic triad occurs in only 50% of cases. 1

Clinical Diagnostic Criteria

The American College of Radiology defines five key clinical criteria that, when all present, approach 100% likelihood of septic arthritis: 1

• Fever >101.3°F (38.5°C) 1
• Erythrocyte sedimentation rate (ESR) ≥40 mm/hour 1
• White blood cell count ≥12,000 cells/mm³ 1
• Inability to bear weight 1
• C-reactive protein >2.0 mg/dL 1

Meeting all five criteria (Kocher criteria) approaches 100% diagnostic certainty, though absence of all criteria does not exclude the diagnosis. 1, 2

Definitive Diagnostic Procedure

Joint aspiration is the gold standard and must be performed in all suspected cases. 3, 1 The procedure should be image-guided (ultrasound, fluoroscopy, or CT) to ensure proper needle placement and maximize diagnostic yield. 3

Synovial Fluid Analysis Requirements:

• Cell count with differential: WBC ≥50,000 cells/mm³ is highly suggestive of septic arthritis 1, 4
• Gram stain: Guides initial antibiotic selection 3, 4
• Culture: Positive in approximately 80% of non-gonococcal cases; essential for definitive diagnosis and antibiotic selection 1, 5

Critical pitfall: A negative synovial fluid culture does NOT exclude infection—if clinical suspicion remains high, consider percutaneous bone biopsy. 1, 5

Imaging Algorithm

The American College of Radiology recommends a structured imaging approach: 3, 1

Initial Imaging:

• Plain radiographs first: Low sensitivity but excludes fractures, tumors, and other conditions 3
• Ultrasound for hip joints: Detects effusions with 95% sensitivity (5% false negative if symptoms <1 day); guides aspiration 1, 2

Advanced Imaging When Indicated:

• MRI with contrast: Use when clinical suspicion remains high despite negative aspiration, or symptoms persist/worsen despite treatment 3, 1
  • Findings include synovial enhancement, joint effusion, and T1 hypointense bone marrow signal 3
  • Distinguishes septic arthritis from transient synovitis: Decreased femoral head enhancement on early post-contrast imaging occurs only in septic arthritis 1, 2
  • Detects concurrent osteomyelitis: Present in >50% of pediatric cases and 30% of adult cases 1, 5, 2

Pathogen Considerations by Age

Understanding likely pathogens guides empiric therapy: 1

• Neonates: Group B streptococcus predominates 1
• Children <4 years: Kingella kingae is common 1
• All ages: Staphylococcus aureus is the most common pathogen overall 1, 6, 4
• Special populations: Salmonella species in sickle cell disease 1, 5

Treatment Initiation

Begin empiric IV antibiotics immediately after obtaining cultures if clinical suspicion is high—do not delay for imaging or culture results. 1, 2

First-Line Empiric Therapy:

• Adults: IV vancomycin 15 mg/kg every 6 hours for MRSA coverage 1, 5
• Pediatrics: IV vancomycin 15 mg/kg/dose every 6 hours (40 mg/kg/day in 4 divided doses) 5
• Alternative if MRSA unlikely: Linezolid 600 mg IV/PO every 12 hours, daptomycin 6 mg/kg IV daily, or teicoplanin 5

Surgical Management:

Immediate surgical drainage (arthrotomy, arthroscopic drainage, or repeated arthrocentesis) is mandatory in all cases. 1, 5 Bacterial proliferation causes irreversible cartilage damage within hours to days. 1, 5

Treatment Duration

• Uncomplicated bacterial arthritis: 3-4 weeks total; recent evidence suggests 2 weeks may be adequate after surgical drainage in select cases (predominantly small joints) 1, 5
• Concomitant osteomyelitis: Longer treatment required (present in up to 30% of children, >50% of pediatric cases on MRI) 1, 5, 2
• Prosthetic joint infections with debridement and retention: 12 weeks superior to 6 weeks 5

Oral antibiotics are not inferior to IV therapy and can be initiated after 2-4 days if the patient is clinically improving, afebrile, and tolerating oral intake. 1, 5

Critical Pitfalls to Avoid

• Never assume single joint involvement: Use large field-of-view imaging when feasible 2
• Always reassess for concurrent osteomyelitis if patient fails to improve after 48-72 hours of appropriate antibiotics and drainage 2
• Monitor CRP and ESR to assess treatment response 1, 5
• In prosthetic joints: Synovial fluid WBC cutoff may be as low as 1,100 cells/mm³ with neutrophil differential >64% 4