Review Question - QID 5675

The clinical picture is most consistent with septic arthritis of the hip given the patient's refusal to bear weight, elevated CRP and WBC count. While the probability of septic arthritis in this scenario is only 40% using the Kocher criteria, inclusion of elevated CRP increases the probability to 74%. A hip aspiration should be performed and synovial fluid sent for cell count with differential, crystals, gram stain and cultures.

Septic arthritis of the hip in the pediatric population requires prompt diagnosis and emergent irrigation and debridement of the hip joint to minimize articular cartilage damage. It can be difficult to differentiate septic arthritis from transient synovitis and Lyme arthritis, especially in Lyme-endemic areas. Clinical indicators including fever and refusal to bear weight, as well as laboratory markers such as WBC count, ESR and CRP can help narrow the diagnosis. Arthrocentesis is indicated when there is a high suspicion for septic arthritis and can help confirm the diagnosis. Synovial fluid analysis usually demonstrates WBC > 50,000/mm3 with PMN > 75% and positive gram stain.

Kocher et al identified four independent multivariate clinical predictors to differentiate between septic arthritis and transient synovitis of the hip in children: 1. History of fever (T > 38.5C), 2. Non-weight-bearing, 3. ESR > 40 mm/h, 4. WBC > 12,000 cells/mm3. The authors developed an evidence-based clinical algorithm to calculate the predicted probability of septic arthritis based on the number of these variables at presentation. Per their algorithm, the probability of septic arthritis is 0.2% with zero predictors, 3.0% with one predictor, 40.0% with two predictors, 93.1% with three predictors and 99.6% with four predictors.

Notably absent from the Kocher criteria is C-reactive protein (CRP) measurement. Singhal et al investigated the use of CRP in a predictive algorithm for differentiating septic arthritis and transient synovitis in the pediatric population. They concluded that CRP > 2 mg/dL was the strongest independent risk factor for septic arthritis with an odds ratio of 81.9. Patients with CRP > 2 mg/dL and refusal to bear weight had a 74% probability of septic arthritis, while patients with neither predictor had a <1% probability of septic arthritis.

Figure A is the ultrasound image of a right hip demonstrating a large effusion. Illustration A shows Figure A labeled with landmarks and highlighting the effusion.

Incorrect Responses:
Answer 1 - NSAIDs are the appropriate management for transient synovitis. Given the high probability of a septic hip joint this would not be correct.
Answer 2 - Doxycycline is the appropriate management for Lyme disease. While the patient did go hiking in a Lyme-endemic area and Lyme disease should remain on the differential, given the high probability of a septic hip joint, hip aspiration is the best next step.
Answer 3 - Intravenous antibiotics should be started after hip aspiration is performed to maximize culture data in non-septic appearing patient.
Answer 5 - This patient's clinical picture is more consistent with a septic hip joint and therefore hip aspiration should be performed.