What is the initial workup and treatment for suspected septic (infectious) arthritis?

Septic Arthritis Workup and Treatment

Initial Diagnostic Approach

Begin with standard radiographs of the affected joint (AP, lateral, and mortise views for ankle; appropriate views for other joints) as the first-line imaging, followed immediately by joint aspiration for synovial fluid analysis—do not delay aspiration to obtain advanced imaging. 1, 2

Clinical Presentation to Assess

• Acute onset of monoarticular joint pain with erythema, heat, and immobility 3
• Fever, chills, and rigors (though these constitutional symptoms have poor sensitivity) 3
• Risk factors: age >80 years, diabetes mellitus, rheumatoid arthritis, recent joint surgery, prosthetic joint, skin infection, immunosuppressive medications 4

Initial Imaging: Plain Radiographs

Obtain standard radiographs first to exclude fractures, tumors, and provide anatomic overview, but recognize that early septic arthritis (<14 days) may show only soft tissue swelling or appear completely normal. 1, 2

Key radiographic findings to look for:

• Joint effusion and soft tissue swelling 1, 2
• Effacement of fat planes 1
• Later stages: joint space narrowing and erosions 2

Definitive Diagnosis: Joint Aspiration

Proceed directly to joint aspiration (with or without imaging guidance) as soon as septic arthritis is suspected—this is the definitive diagnostic procedure and should not be delayed. 1, 5

Synovial Fluid Analysis Required

• Cell count with differential: WBC ≥50,000 cells/mm³ suggests septic arthritis (in non-prosthetic joints) 5, 3
  • For prosthetic joints, threshold is much lower: ≥1,100 WBC/mm³ with neutrophils >64% 3
• Gram stain: guides initial antibiotic selection 3, 4
• Culture: positive in ~80% of non-gonococcal cases; essential for definitive pathogen identification 5, 3
• Crystal analysis: to exclude gout/pseudogout (though these can coexist with infection) 1, 3

Image-Guided Aspiration Considerations

• Use ultrasound or fluoroscopy guidance if joint is not distended or to target specific compartments 1
• Ultrasound particularly useful for hip joints to detect effusions and guide aspiration 5
• Image guidance reduces vascular complications and nerve injury 1

Laboratory Workup

• Blood cultures before initiating antibiotics 6
• Serum inflammatory markers: ESR and CRP (helpful but nonspecific; use for monitoring treatment response) 5, 6, 4
• Complete blood count with differential 6

Advanced Imaging (When Indicated)

MRI with IV contrast is the next appropriate study if:

• Radiographs are normal but clinical suspicion remains high 1, 2
• Need to assess extent of infection, soft tissue abscesses, or concomitant osteomyelitis 1, 2
• Planning surgical debridement 1

MRI rated 9/9 for appropriateness by ACR for suspected septic arthritis after initial radiographs 2

Do not obtain MRI before joint aspiration unless it will guide the aspiration procedure. 1

Immediate Treatment

Empiric Antibiotic Therapy

Start IV vancomycin immediately after obtaining cultures to cover MRSA, which is the most common pathogen and increasingly prevalent. 5, 6, 7

Antibiotic selection based on Gram stain:

• Gram-positive cocci: Vancomycin 3
• Gram-negative cocci: Ceftriaxone 3
• Gram-negative rods: Ceftazidime 3
• Negative Gram stain with high clinical suspicion: Vancomycin plus ceftazidime or aminoglycoside 3

Surgical Management

Joint drainage is mandatory—perform arthrocentesis, arthroscopic drainage, or open surgical debridement. 5, 7

• Surgical emergencies require immediate debridement with intraoperative cultures 5, 6
• Non-emergency cases: aspiration first, then surgical drainage if needed 5

Definitive Antibiotic Therapy

Once culture results available:

• MRSA confirmed: Continue IV vancomycin; consider adding rifampin 600 mg daily or 300-450 mg twice daily for better bone/biofilm penetration 5
• MSSA: Switch to nafcillin, oxacillin, or cefazolin 5
• Polymicrobial infection: Dual antibiotic coverage mandatory (e.g., linezolid for MRSA plus ciprofloxacin for Pseudomonas) 5

Treatment Duration

• Uncomplicated bacterial arthritis: 3-4 weeks total 5, 7
• With concomitant osteomyelitis: 6 weeks 7
• Prosthetic joint infections: 12 weeks shows better outcomes 5
• Oral antibiotics: Can transition after clinical improvement; not inferior to IV therapy in most cases 4

Special Pathogen Considerations

• Children <4 years: Consider Kingella kingae 5, 8
• Sickle cell disease: Consider Salmonella species 5
• Sexually active adults: Consider Neisseria gonorrhoeae 4
• Endemic areas: Consider Borrelia burgdorferi (Lyme), fungal infections 4

Monitoring Treatment Response

• Follow CRP and ESR to assess response 5, 6
• Resolution of SIRS criteria before discharge if hospitalized 6
• Downtrending inflammatory markers necessary before transitioning to outpatient care 6

Critical Pitfalls to Avoid

• Never delay joint aspiration to obtain advanced imaging—early diagnosis and treatment are essential to prevent permanent joint damage 1, 9
• Do not rely on normal radiographs to exclude infection—early septic arthritis frequently has normal or minimal findings 1, 2
• Negative synovial fluid culture does not rule out infection—consider percutaneous bone biopsy if clinical suspicion remains high 5
• Watch for concomitant osteomyelitis—occurs in up to 30% of pediatric cases and requires longer treatment 5, 6
• Patients meeting SIRS criteria require hospital admission—risk of rapid deterioration and need for IV antibiotics and close monitoring 6