What are the signs and diagnosis of septic arthritis?

Signs and Diagnosis of Septic Arthritis

The definitive diagnosis of septic arthritis requires joint aspiration with direct demonstration of bacteria in synovial fluid or positive culture, combined with clinical findings of acute joint pain, erythema, warmth, swelling, limited mobility, and characteristic night pain or pain at rest. 1

Clinical Presentation

• Key clinical features:

  • Acute onset joint pain
  • Erythema and warmth over the affected joint
  • Joint swelling
  • Limited range of motion
  • Night pain or pain at rest (highly characteristic of infection)
  • Fever (may be absent in up to 40% of cases)

• In pediatric patients, the Kocher criteria are recommended:

  • Fever >101.3°F
  • ESR ≥40 mm/hour
  • White blood cell count ≥12,000 cells/mm³
  • Inability to bear weight 1

Laboratory Assessment

• Serum inflammatory markers:

  • Elevated C-reactive protein (CRP) >2.0 mg/dL
  • Elevated erythrocyte sedimentation rate (ESR) ≥40 mm/hour
  • Elevated white blood cell count (though less specific) 1

• Synovial fluid analysis (critical for diagnosis):

  • White blood cell count >50,000 cells/mm³ with >90% neutrophils is highly suggestive of septic arthritis in native joints
  • Neutrophil-to-lymphocyte ratio (NLR) >25 has 78% sensitivity and 81% specificity 1
  • Lower thresholds for prosthetic joints: >1,100 cells/mm³ with >64% neutrophils 1
  • Gram stain (positive in only 50-60% of cases)
  • Culture (gold standard but may take days)
  • Alpha-defensin testing (97% sensitivity, 96% specificity) 1

Imaging Studies

• Plain radiographs:

  • First-line imaging to identify soft tissue swelling, joint effusion, gas
  • Helps exclude fractures or tumors 1
  • May appear normal early in the disease

• Ultrasound:

  • Highly sensitive for detecting joint effusion
  • Guides joint aspiration
  • Particularly valuable in children (no radiation exposure) 1, 2
  • Absence of hip joint effusion virtually excludes septic arthritis (5% false negative rate) 2

• MRI with contrast:

  • Recommended when joint aspiration is inconclusive or technically difficult
  • High sensitivity for detecting concurrent osteomyelitis
  • Decreased femoral head enhancement on early post-contrast imaging is characteristic of septic arthritis 2, 1
  • Can identify infection beyond the joint space 2

• CT with IV contrast:

  • Alternative when MRI is contraindicated
  • Evaluates for periosteal reaction, bone erosions, or suspected soft tissue gas 1

Diagnostic Algorithm

1. Initial assessment:

   • Clinical evaluation for signs of joint infection
   • Serum inflammatory markers (CRP, ESR, WBC)
   • Plain radiographs of affected joint

2. Joint aspiration (arthrocentesis):

   • Essential for definitive diagnosis
   • Should be performed before starting antibiotics
   • Often guided by ultrasound or fluoroscopy 1
   • Send for:
     • Cell count with differential
     • Gram stain
     • Culture (aerobic and anaerobic)
     • Crystal analysis (to rule out concurrent crystal arthropathy)
     • Additional tests: leukocyte esterase, alpha-defensin

3. Advanced imaging:

   • MRI if aspiration is inconclusive or to evaluate for concurrent osteomyelitis
   • Ultrasound for detecting effusions and guiding aspiration
   • 3-phase bone scan for suspected multifocal infection or when MRI is contraindicated 1

Common Pitfalls and Caveats

• Delayed diagnosis can result in permanent joint damage or mortality - maintain high clinical suspicion 3
• False negatives can occur in patients who have received antibiotics prior to aspiration 1
• "Dry tap" (absence of fluid) does not rule out infection 1
• Crystal arthropathy may coexist with septic arthritis - the presence of crystals does not exclude infection 1
• Prosthetic joint infections require a lower threshold for diagnosis and different management approach 1
• Concurrent osteomyelitis is common in septic arthritis and may be missed without appropriate imaging 2, 1
• Systemic sepsis is associated with poorer outcomes and higher rates of recurrent infection 4

Early diagnosis and prompt treatment are essential to prevent irreversible joint destruction and potentially fatal outcomes. The combination of clinical assessment, laboratory tests, and appropriate imaging provides the most accurate diagnostic approach.