What is the diagnostic approach for septic arthritis?

Diagnostic Approach for Septic Arthritis

The definitive diagnosis of septic arthritis requires joint aspiration with synovial fluid analysis, which should be performed promptly when clinical suspicion exists, as delayed diagnosis can lead to permanent joint damage and significant morbidity. 1

Clinical Evaluation

Key Clinical Features to Assess:

• Acute onset of monoarticular joint pain
• Joint swelling, erythema, warmth
• Limited range of motion and joint immobility
• Constitutional symptoms (fever, chills) - though these are present in only about 50% of cases 2, 3

High-Risk Populations:

• Age >80 years
• Diabetes mellitus
• Rheumatoid arthritis
• Recent joint surgery
• Prosthetic joints
• Skin infection
• Immunosuppressive medication use 2
• End-stage renal disease on dialysis 4

Diagnostic Algorithm

Step 1: Initial Laboratory Tests

• Complete blood count
• Erythrocyte sedimentation rate (ESR) - sensitivity 96% 5
• C-reactive protein (CRP) - elevated >2.0 mg/dL is predictive 1
• Blood cultures (before antibiotics if possible)

Step 2: Initial Imaging

• Plain radiographs of the affected joint
  • Helps identify soft tissue swelling, joint effusion, gas
  • Excludes fractures or tumors as alternative diagnoses
  • Establishes baseline for subsequent imaging 1, 6

Step 3: Joint Aspiration (Gold Standard)

• Ultrasound-guided joint aspiration should be performed when:
  • Radiographs show effusion
  • Radiographs are normal but clinical suspicion remains high 6
• Send synovial fluid for:
  • Cell count with differential
  • Gram stain
  • Culture and sensitivity
  • Crystal analysis (to rule out concurrent crystal arthropathy) 6

Step 4: Synovial Fluid Analysis Interpretation

• Synovial WBC count interpretation:

  • 50,000 cells/mm³ in patients without prior antibiotics (sensitivity 96%, specificity 95%) 7

  • 16,000 cells/mm³ in patients who received antibiotics before aspiration (sensitivity 82%, specificity 76%) 7

  • 90% neutrophils in patients who received antibiotics 7

  • 83% neutrophils in patients without prior antibiotics 7

Step 5: Additional Imaging (if needed)

• MRI with contrast is indicated when:
  • Joint aspiration is inconclusive or technically difficult
  • Concern for adjacent osteomyelitis exists
  • Need to evaluate extent of infection beyond joint space 1, 6
• CT with IV contrast if MRI is contraindicated 6
• 3-phase bone scan may be useful for:
  • Suspected multifocal infection
  • Infection associated with orthopedic hardware
  • Patients with contraindications to MRI 1

Important Caveats and Pitfalls

• Do not rely solely on laboratory tests to rule out septic arthritis, as WBC, ESR, and synovial fluid WBC counts can be highly variable 5
• Do not delay joint aspiration if clinical suspicion is high, even with normal inflammatory markers 3
• Consider concurrent infections such as osteomyelitis, which is common in pediatric patients 1, 6
• Remember that crystal arthropathy and septic arthritis can coexist 6, 3
• Lower threshold for septic arthritis diagnosis in prosthetic joints - intraarticular cutoff values may be as low as 1,100 WBC/mm³ with neutrophil differential >64% 3
• Begin empiric antibiotics after synovial fluid has been obtained if clinical suspicion remains high 2

Special Considerations for Pediatric Patients

• The Kocher criteria help distinguish septic arthritis from transient synovitis in pediatric hip pain:
  • Fever >101.3°F
  • ESR ≥40 mm/hour
  • WBC count ≥12,000 cells/mm³
  • Inability to bear weight
  • Meeting all four criteria has nearly 100% likelihood of septic arthritis 1

Remember that septic arthritis is considered an orthopedic emergency, and prompt diagnosis and treatment are essential to prevent permanent joint damage, which can significantly impact morbidity, mortality, and quality of life.