What is the recommended work‑up and management for a patient presenting with knee pain, swollen lymph nodes, and suspected Group B Streptococcus septic arthritis?

Suspected Group B Streptococcus Septic Arthritis with Knee Pain and Lymphadenopathy

Perform immediate knee joint aspiration before administering antibiotics, send synovial fluid for cell count, Gram stain, and culture, obtain blood cultures, and start IV vancomycin empirically after specimens are collected. 1, 2

Immediate Diagnostic Steps

Joint Aspiration (Gold Standard)

• Arthrocentesis must be performed emergently before any antibiotics are given, as antibiotic administration reduces culture sensitivity from 79% to 28% and microscopy sensitivity from 58% to 12%. 3
• Knee aspiration can be performed at bedside without imaging guidance, unlike hip joints which require ultrasound. 1, 4
• Send synovial fluid for: white blood cell count with differential (≥50,000 cells/mm³ suggests septic arthritis), Gram stain, aerobic and anaerobic cultures, and crystal analysis to exclude coexistent gout. 5, 1, 2
• A "dry tap" does not exclude infection—if aspiration yields no fluid but clinical suspicion remains high, proceed to imaging-guided aspiration or MRI. 5

Laboratory Testing

• Obtain blood cultures before antibiotics, as Group B Streptococcus bacteremia commonly accompanies septic arthritis. 1, 6
• Check ESR (≥40 mm/hour), CRP (>2.0 mg/dL), and WBC count (≥12,000 cells/mm³)—meeting all criteria approaches 100% likelihood of septic arthritis. 1, 4
• Consider α-defensin testing from synovial fluid if available (97% sensitivity, 96% specificity), though this should not delay treatment. 5

Initial Imaging Algorithm

Plain Radiographs First

• Obtain knee radiographs immediately to exclude fractures, tumors, and other bony pathology, though they are normal in early infection (<14 days) and have low sensitivity. 1, 2
• Look for joint effusion, soft tissue swelling, or gas in soft tissues—these findings warrant immediate aspiration. 1

Ultrasound for Effusion Detection

• Ultrasound detects joint effusions as small as 1 mL with only 5% false-negative rate (if symptoms >24 hours), and can guide aspiration if needed. 1, 4
• Critical pitfall: Ultrasound performed within 24 hours of symptom onset may yield false-negative results. 1

Empiric Antibiotic Therapy

Start After Cultures Obtained

• IV vancomycin 15 mg/kg every 6 hours is first-line for MRSA coverage in adults, as Staphylococcus aureus remains the most common pathogen overall. 1
• For Group B Streptococcus specifically (common in adults with recent gynecologic procedures or immunocompromise), vancomycin provides adequate coverage while awaiting culture results. 1, 6
• Alternatives include linezolid, daptomycin, or teicoplanin if vancomycin is contraindicated. 1

Treatment Duration

• Total antibiotic duration: 3-4 weeks for uncomplicated bacterial arthritis, though recent evidence suggests 2 weeks may be adequate after surgical drainage in select cases. 1
• Transition to oral antibiotics after 2-4 days if clinically improving, afebrile, and tolerating oral intake—oral therapy is not inferior to IV. 1

Surgical Intervention

Indications for Drainage

• Immediate surgical drainage is mandatory because bacterial proliferation causes irreversible cartilage damage within hours to days. 1, 2
• Repeated needle aspiration alone fails in 46% of cases—arthroscopic debridement with suction drains is preferred. 7
• Surgical drainage is especially critical when symptoms persist >7 days, patient has severe sepsis, or prosthetic joint involvement. 1

Advanced Imaging: When to Order MRI

MRI Indications

• Order MRI with contrast when: clinical suspicion remains high despite negative aspiration, concern for concurrent osteomyelitis (present in >50% of pediatric cases and up to 30% in adults), or need to assess soft tissue abscess. 1, 2
• MRI has 82-100% sensitivity and 75-96% specificity for septic arthritis, and decreased femoral head enhancement on early post-contrast imaging is specific for septic arthritis versus transient synovitis. 1
• Do not delay definitive treatment waiting for MRI—aspiration and antibiotics take priority. 1

Lymphadenopathy Considerations

• Swollen lymph nodes in the setting of knee septic arthritis suggest systemic spread or regional lymphatic involvement from joint infection. 8
• Ensure blood cultures are obtained to detect bacteremia, as Group B Streptococcus commonly causes both joint infection and bacteremia simultaneously. 6
• If lymphadenopathy is extensive or patient appears septic, consider broader imaging (CT chest/abdomen/pelvis) to evaluate for endocarditis or other metastatic foci, particularly in immunocompromised patients. 6

Monitoring and Follow-Up

• Monitor CRP and ESR to assess treatment response—these should trend downward with appropriate therapy. 1
• Check vancomycin trough levels to adjust dosing and avoid toxicity. 1
• If symptoms worsen or persist despite treatment, obtain repeat MRI—this leads to management changes in 21% of cases and may reveal concurrent osteomyelitis requiring extended therapy. 1

Critical Pitfalls to Avoid

• Never start antibiotics before obtaining joint fluid and blood cultures—this dramatically reduces diagnostic yield. 3
• Negative synovial fluid culture does not exclude infection, especially if antibiotics were given first—consider percutaneous bone biopsy if clinical suspicion remains high. 1, 2
• Do not assume crystal arthropathy excludes septic arthritis—73% of reported cases of coexistent sepsis and gout had positive cultures. 1
• Group B Streptococcus septic arthritis is increasingly recognized in non-pregnant adults, particularly after gynecologic procedures (including routine Pap smears), so maintain high clinical suspicion in this population. 6