What is the recommended workup and treatment for a patient with suspected septic arthritis?

Septic Arthritis Workup and Treatment

Immediate Diagnostic Approach

Septic arthritis is an orthopedic emergency requiring immediate joint aspiration for synovial fluid analysis, followed by prompt surgical drainage and empiric IV vancomycin after cultures are obtained. 1, 2

Clinical Presentation and Risk Assessment

• Suspect septic arthritis in patients with acute monoarticular joint pain, swelling, and fever, though the classic triad occurs in only ~50% of cases 3
• Key diagnostic criteria include: fever >101.3°F, ESR ≥40 mm/hour, WBC ≥12,000 cells/mm³, inability to bear weight, and CRP >2.0 mg/dL—meeting all criteria approaches 100% likelihood of septic arthritis 3
• High-risk populations include patients >80 years, diabetes mellitus, rheumatoid arthritis, recent joint surgery, prosthetic joints, skin infections, and immunosuppressive medication use 4

Initial Imaging Strategy

Start with ultrasound for hip joints, plain radiographs for other joints, followed by MRI if clinical suspicion remains high despite negative aspiration. 5, 2, 3

• Ultrasound is the initial imaging modality of choice for hip joints to detect effusions (can detect as small as 1 mL) and guide aspiration, with only 5% false-negative rate if symptoms <1 day 5, 3
• Absence of hip joint effusion on ultrasound virtually excludes septic arthritis 3
• Plain radiographs should be obtained to exclude fractures and tumors, though they have low sensitivity for early infection and may be normal in <14 days 2, 3
• MRI has 82-100% sensitivity and 75-96% specificity and should be ordered when clinical suspicion remains high despite negative aspiration, concern for concurrent osteomyelitis exists, or soft tissue abscess needs assessment 3
• Decreased femoral head enhancement on early post-contrast MRI reliably distinguishes septic arthritis from transient synovitis 5, 2

Definitive Diagnosis: Joint Aspiration

Joint aspiration is the definitive diagnostic procedure and must be performed before initiating antibiotics, with US guidance required for hip joints. 1, 2, 3

Synovial Fluid Analysis Thresholds

• Synovial fluid WBC ≥50,000 cells/mm³ is suggestive of septic arthritis in native joints 1, 2, 6
• Critical caveat: If antibiotics were given within 2 weeks before aspiration, lower the diagnostic threshold to >16,000 cells/mm³ (sensitivity 82%, specificity 76%) 7
• For patients without prior antibiotics, the optimal cutoff is >33,000 cells/mm³ (sensitivity 96%, specificity 95%) 7
• Neutrophil percentage >90% supports septic arthritis in antibiotic-exposed patients; >83% in antibiotic-naive patients 7
• For prosthetic joint infections, the threshold is much lower: ≥1,100 WBC/mm³ with neutrophil differential >64% 6

Culture and Gram Stain

• Synovial fluid culture is positive in ~80% of non-gonococcal septic arthritis cases 1, 2
• Obtain blood cultures before initiating antibiotics 2
• Gram stain results should guide initial antibiotic selection (sensitivity 0.76, specificity 0.96) 3, 6
• Critical pitfall: Negative synovial fluid culture does not exclude infection—consider percutaneous bone biopsy if clinical suspicion remains high 1, 2, 3
• Crystal analysis must be performed even if infection is suspected, as crystals and septic arthritis can coexist in 73% of reported cases 3

Surgical Management

Immediate surgical drainage is mandatory in all cases of septic arthritis—arthrotomy, irrigation, and debridement are the standard interventions. 1, 2

• Surgical drainage should be performed through arthrocentesis, arthroscopic drainage, or open surgical debridement 2
• Surgical drainage is mandatory when symptoms persist >7 days, patient has severe sepsis, or hip/shoulder involvement exists, as repeated needle aspiration alone fails in 46% of cases 3
• For prosthetic joint infections, device removal is recommended 1, 3
• If prosthetic device cannot be removed, chronic suppression with fluconazole 400 mg daily (if susceptible) is recommended 1

Empiric Antibiotic Therapy

Start IV vancomycin 15 mg/kg every 6 hours immediately after obtaining cultures to cover MRSA, which is the most common pathogen. 1, 2, 3

Adult Empiric Regimens

• IV vancomycin 30-60 mg/kg/day in 2-4 divided doses or 15 mg/kg IV every 6 hours is first-line for adults 2, 3
• Alternative empiric options if MRSA less likely: linezolid 600 mg IV/PO every 12 hours, daptomycin 6 mg/kg IV daily, or teicoplanin 6-12 mg/kg IV every 12 hours for 3 doses then daily 2
• If Gram stain is negative but strong clinical suspicion exists, use vancomycin plus ceftazidime or an aminoglycoside 6

Pediatric Empiric Regimens

• IV vancomycin 15 mg/kg/dose every 6 hours (40 mg/kg/day in 4 divided doses) for children 2, 3
• Clindamycin 10-13 mg/kg/dose IV every 6-8 hours is an alternative if local clindamycin resistance is low 1, 2
• In children <4 years, consider Kingella kingae as a causative organism 1, 2, 3
• In neonates, Group B streptococcus predominates 3
• In patients with sickle cell disease, consider Salmonella species 1, 2

Definitive Culture-Directed Therapy

Methicillin-Sensitive Staphylococcus aureus (MSSA)

• Switch to nafcillin or oxacillin 1-2 g IV every 4 hours, OR cefazolin 1 g IV every 8 hours, OR clindamycin 600 mg IV every 8 hours (if penicillin allergic) 2

Methicillin-Resistant Staphylococcus aureus (MRSA)

• Continue vancomycin as primary therapy 1, 2
• Consider adding rifampin 600 mg PO daily or 300-450 mg PO twice daily for enhanced bone and biofilm penetration 1, 2

Streptococcal Infections

• Penicillin G 20-24 million units IV daily continuous infusion or divided doses, OR ceftriaxone 1-2 g IV every 24 hours 2

Polymicrobial Infections

• Dual antibiotic coverage is mandatory when multiple organisms are identified (e.g., linezolid for MRSA plus ciprofloxacin for Pseudomonas aeruginosa) 2

Fungal Septic Arthritis (Candida)

• Fluconazole 400 mg daily for 6 weeks OR an echinocandin (caspofungin 50-70 mg daily, micafungin 100 mg daily, or anidulafungin 100 mg daily) for 2 weeks followed by fluconazole 400 mg daily for at least 4 weeks 1, 2
• All patients with candidemia should have dilated retinal examination within the first week to establish if endophthalmitis is present 1

Route and Duration of Antibiotic Therapy

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. 2, 3, 4

Treatment Duration

• 3-4 weeks total for uncomplicated bacterial arthritis 1, 2, 3
• Recent evidence suggests 2 weeks may be adequate after surgical drainage in select cases (predominantly small joints) 2
• For concomitant osteomyelitis (occurs in up to 30% of children), longer treatment is required 1, 2
• For prosthetic joint infections with debridement and implant retention: 12 weeks is superior to 6 weeks 1, 2
• For hip prostheses with one-stage or two-stage exchange: 3 months total 2
• For knee prostheses with one-stage or two-stage exchange: 6 months total 2

Oral Antibiotic Options for MRSA (after initial IV therapy)

• Linezolid 600 mg PO every 12 hours 2
• TMP-SMX (trimethoprim 4 mg/kg/dose) PO every 8-12 hours plus rifampin 600 mg PO daily 2
• Fusidic acid 500 mg PO every 8 hours or 750 mg every 12 hours plus rifampin 2

Monitoring and Follow-Up

• Monitor CRP and ESR to assess treatment response 2, 3
• Monitor vancomycin trough levels to adjust for toxicity 2
• Be vigilant for drug interactions and adverse effects, especially in elderly patients 2
• Repeat MRI for worsening or persistent symptoms, which results in management changes in 21% of cases 3

Special Considerations for Concurrent Osteomyelitis

Over 50% of pediatric patients with septic arthritis have infection beyond the joint space on MRI, requiring assessment for concurrent osteomyelitis. 3

• Five variables predict adjacent infection: CRP >13.8 mg/L, absolute neutrophil count >8.6×10³ cells/mL, platelet level <314 cells/mL, duration of symptoms >3 days, and age >3.6 years 5
• Presence of at least 3 risk factors indicates high risk for adjacent infection (sensitivity 90%, positive predictive value 80%) 5
• Femoral neck aspiration at time of incision and drainage improves diagnosis of concurrent osteomyelitis compared with preoperative MRI alone 5
• Concurrent osteomyelitis occurs in ~58% of pediatric elbow septic arthritis cases 3

Management of Persistent Symptoms

• For persistent or recurrent joint swelling after oral antibiotics, re-treat with another 4-week course of oral antibiotics or 2-4 weeks of IV ceftriaxone 1, 2
• Arthroscopic synovectomy may reduce duration of joint inflammation in cases of persistent synovitis with significant pain or functional limitation 1, 2