Acute monoarthritis and septic arthritis — branching viva

Branching framework

Open with the one-sentence problem representation. This is a febrile child with an acutely hot, held joint, and septic arthritis is the must-not-miss diagnosis until excluded by the synovial fluid. State the urgency aloud — cartilage destruction begins within hours, and the septic hip threatens the femoral head blood supply — before you discuss anything else. The examiner is listening for whether you reach for the aspiration and the theatre before you reach for the antibiotic. ^{[1] [3]}

Apply the Kocher criteria to estimate the probability. The four predictors are a fever above 38.5 degrees, an inability to bear weight, an ESR above 40, and a white cell count above 12,000, with the Caird addition of a CRP above 20 milligrams per litre. State the rising probability with each predictor, and be ready for the calibration probe — the Caird prospective study found the original Kocher probabilities may overestimate, so the criteria sharpen suspicion but never rule out sepsis. A child with two or more predictors warrants urgent aspiration. ^{[1] [3]}

Branch to the single most important investigation. The joint aspiration, under ultrasound guidance for the hip, sending the synovial fluid for a cell count above 50,000, a Gram stain, and culture, with blood-culture-bottle inoculation for Kingella kingae. State that the aspiration precedes the antibiotic, always, because an antibiotic given first sterilises the specimen. If the examiner probes a culture-negative result in the under-four, name Kingella kingae and the oropharyngeal swab PCR. ^{[1] [7]}