A 5-year-old child presents with acute septic arthritis of the hip. Which laboratory threshold most accurately differentiates septic arthritis from transient synovitis in the Kocher criteria?
- A WBC > 12,000/mm³, ESR > 40 mm/hr, fever >38.5°C, and inability to weight-bear (4 criteria predict >99% probability) ✓
- B CRP > 2 mg/dL alone
- C Effusion on ultrasound plus fever
- D Plain X-ray showing hip joint space widening
Explanation
Kocher criteria differentiate septic arthritis from transient synovitis of the hip: (1) Non-weight-bearing, (2) ESR >40 mm/hr, (3) Fever >38.5°C, (4) Serum WBC >12,000/mm³. Each criterion present doubles the probability of septic arthritis; all 4 present = >99% probability. Dodwell's modification adds CRP >2 mg/dL as a 5th criterion, further improving specificity. Transient synovitis is the commonest cause of acute hip pain in children and is self-limiting, while septic arthritis is a surgical emergency requiring urgent arthrotomy and washout. Ultrasound demonstrates effusion in both but cannot distinguish the two.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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