What is the diagnosis and treatment of septic arthritis?

Diagnosis and Treatment of Septic Arthritis

Septic arthritis is an orthopedic emergency requiring immediate joint aspiration for synovial fluid analysis (Gram stain, culture, cell count) followed by urgent surgical drainage and empiric intravenous antibiotics targeting MRSA, with vancomycin as first-line therapy in adults. 1, 2

Diagnostic Approach

Clinical Presentation

• Acute monoarticular joint pain, swelling, erythema, heat, and immobility are the hallmark features, though constitutional symptoms like fever are poorly sensitive and present in only 39.7% of cases at hospital admission 3, 4
• The knee and hip are most commonly affected joints (54.3% knee involvement) 5, 4
• Risk factors include age >80 years, diabetes mellitus (19.1%), rheumatoid arthritis (17.6%), recent joint surgery, prosthetic joints, skin infection, and immunosuppressive medications 6, 4

Laboratory Evaluation

Synovial Fluid Analysis (Gold Standard):

• Synovial fluid white blood cell count ≥50,000 cells/mm³ is highly suggestive of septic arthritis (sensitivity 0.76, specificity 0.96) 5, 1, 3
• For prosthetic joints, the threshold is much lower: ≥1,100 WBC/mm³ with >64% neutrophils 3
• Gram stain guides initial antibiotic selection (positive in 73-76% of cases) 5
• Synovial fluid culture is positive in approximately 80% of non-gonococcal cases and remains the definitive diagnostic test 1, 3

Critical Caveat: The presence of monosodium urate (MSU) or calcium pyrophosphate (CPPD) crystals does NOT exclude septic arthritis—coexistent gout and septic arthritis occur in 4% of cases 5. If synovial TNC >50,000 cells, PMN >90%, and serum CRP >10 mg/dL in a crystal-positive joint, there is a 7.7-fold increased risk of concurrent septic arthritis requiring urgent surgical intervention 7.

Serum Markers:

• C-reactive protein elevated in 97.1% of cases (>10 mg/dL increases infection odds 3.2-fold) 4, 7
• Erythrocyte sedimentation rate elevated in 75% of cases 4
• Peripheral leukocytosis present in only 45.6% of patients 4

Imaging

• Ultrasound is highly sensitive for detecting joint effusions and can guide aspiration, with a false-negative rate of only 5% 5
• MRI has 82-100% sensitivity and 75-96% specificity for detecting osteomyelitis, which occurs concomitantly in up to 30% of pediatric cases and requires longer treatment 5, 1
• Plain radiographs have low sensitivity for early infection but may show soft-tissue swelling 5

Treatment Algorithm

Immediate Management (Within Hours)

1. Joint Aspiration:

• Perform arthrocentesis immediately for Gram stain, culture, cell count with differential, and crystal analysis 5, 1
• Do not delay aspiration if septic arthritis is suspected—missed or delayed diagnosis results in longer hospitalizations and worse outcomes 5
• Image-guided aspiration (ultrasound, fluoroscopy, or CT) improves diagnostic yield, particularly for hip joints 5

2. Surgical Drainage:

• Immediate surgical debridement with arthrotomy, irrigation, and debridement is essential and should not be delayed 5, 1, 2
• Surgical drainage is indicated in ALL cases of confirmed septic arthritis 1
• For prosthetic joint infections, device removal is recommended; if removal is not possible, chronic suppression with fluconazole 400 mg daily (if susceptible) is required 1

3. Empiric Antibiotic Therapy:

Start immediately after obtaining cultures 1, 2:

Adults:

• IV vancomycin is first-line therapy for MRSA coverage (most common pathogen: Staphylococcus aureus) 1, 2, 3
• If Gram stain shows gram-positive cocci: vancomycin 3
• If Gram stain shows gram-negative cocci: ceftriaxone 3
• If Gram stain shows gram-negative rods: ceftazidime 3
• If Gram stain is negative but clinical suspicion is high: vancomycin PLUS ceftazidime or an aminoglycoside 3

Children:

• IV vancomycin for MRSA coverage 5, 1, 2
• Clindamycin 10-13 mg/kg/dose IV every 6-8 hours is an alternative if local clindamycin resistance is <10% 1, 2
• Consider Kingella kingae in children <4 years of age 5, 1, 8
• Consider Salmonella species in patients with sickle cell disease 5, 1, 8

Adjunctive Therapy:

• Adding rifampin 600 mg daily (or 300-450 mg twice daily) to vancomycin for MRSA infections improves outcomes due to excellent bone and biofilm penetration 1, 2

Transition to Oral Therapy

Criteria for transition 2:

• Clinical improvement documented
• No ongoing bacteremia or sepsis
• Afebrile for 24-48 hours

Oral antibiotic options for MRSA 2:

• Clindamycin 600 mg PO three times daily
• Linezolid 600 mg PO twice daily
• Trimethoprim-sulfamethoxazole 3.5-4.0 mg/kg/dose PO every 8-12 hours
• Consider adding rifampin for enhanced bone penetration 2

Duration of Therapy

Standard cases:

• 3-4 weeks total antibiotic duration for uncomplicated septic arthritis 1, 2, 6

Extended therapy:

• Prosthetic joint infections: 4-6 weeks minimum (12 weeks shows better outcomes than 6 weeks) 1, 2
• Concomitant osteomyelitis: longer treatment required (up to 30% of pediatric cases) 1, 2
• Fungal (Candida) septic arthritis: fluconazole 400 mg daily for 6 weeks OR echinocandin for 2 weeks followed by fluconazole 400 mg daily for ≥4 weeks 1

Persistent symptoms:

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

Pathogen-Specific Considerations

Most common organisms 3, 6:

• Staphylococcus aureus (>80% of non-gonococcal cases)
• Streptococcal species
• Gram-negative bacilli
• Neisseria gonorrhoeae (consider based on sexual history)
• Borrelia burgdorferi (Lyme arthritis in endemic areas)

Special populations 5, 1, 8:

• Neonates: Group B streptococcus
• Children <4 years: Kingella kingae
• Sickle cell disease: Salmonella species

Critical Pitfalls to Avoid

1. Never delay joint aspiration and surgical drainage while awaiting laboratory results—septic arthritis causes irreversible cartilage damage within hours to days 5, 1

2. Do not rule out septic arthritis based on the presence of crystals—perform full septic workup including synovial fluid Gram stain and culture even if MSU crystals are identified 5

3. Do not transition to oral antibiotics if the patient has ongoing bacteremia, sepsis, or lacks clinical improvement 2

4. Do not use oral antibiotics alone without prior surgical drainage—evacuation of purulent material is mandatory 2, 3

5. Negative synovial fluid culture does not exclude infection—41.2% of clinically diagnosed septic arthritis cases have negative cultures but similar clinical characteristics and require full treatment 4

6. For all patients with candidemia, perform dilated retinal examination within the first week to evaluate for endophthalmitis 1

7. In polymicrobial infections (which do occur), ensure dual antibiotic coverage addressing all identified organisms 1, 2