In a pediatric patient with suspected transient synovitis versus septic arthritis causing limp, which investigation helps differentiate the two conditions?

• A. Ultrasound
• B. Measurement of serum inflammatory markers
• C. MRI
• D. Joint aspiration

Answer: (B)

Evaluating systemic inflammation through blood tests helps distinguish septic arthritis from transient synovitis in a child with a limp. Infections in a joint provoke a stronger inflammatory response, so markers like C-reactive protein and ESR tend to be higher with septic arthritis than with transient synovitis. This makes serum inflammatory markers a fast, noninvasive way to triage: a markedly elevated CRP or ESR increases suspicion for septic arthritis and supports moving toward urgent joint aspiration to obtain synovial fluid for definitive analysis. If these markers are normal or only mildly elevated, transient synovitis becomes more likely, though infection cannot be ruled out solely on blood tests. While imaging can reveal an effusion, and MRI can detail joint structures, neither confirms infection as reliably as direct analysis of the joint fluid; blood inflammatory markers help guide the decision to pursue that next step.