A 7-year-old child presents with fever, limp, and refusal to weight bear. WBC is 18,000/mm3, ESR 65 mm/hr, CRP 85 mg/L. Hip ultrasound shows a joint effusion. The single most important investigation to differentiate septic arthritis from transient synovitis is:
- A MRI of the hip with gadolinium
- B Hip aspiration with synovial fluid WBC count, glucose, protein, Gram stain, and culture ✓
- C Kocher criteria prediction score assessment alone
- D Technetium bone scan
Correct answer: B. Hip aspiration with synovial fluid WBC count, glucose, protein, Gram stain, and culture
Explanation
While Kocher criteria (fever > 38.5°C, non-weight-bearing, ESR > 40, WBC > 12,000) and subsequently modified Caird criteria (adding CRP > 20) predict septic arthritis probability, definitive differentiation requires hip aspiration. Synovial fluid WBC > 50,000/mm3 with polymorphonuclear predominance, positive Gram stain, low glucose, and positive culture confirm septic arthritis. Transient synovitis produces lower WBC (< 50,000), normal glucose, and negative culture. Bone scan and MRI do not directly analyse synovial fluid.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.