Background Laboratory tests for cases of suspected orthopaedic infection usually include measurement of both the erythrocyte sedimentation rate (ESR) and the C reactive protein (CRP). This has a time and cost implication for the laboratories. The measurement of both also requires a larger volume blood sample, which can be difficult to obtain.

Objective To assess if the measurement of either CRP or ESR individually has a likelihood ratio equivalent to that of the measurement of both.

Setting Emergency Department of a Paediatric Teaching Hospital.

Study design Retrospective study of Case Notes for patients attending the Emergency Department with a complaint of atraumatic limb pain and in whom both ESR and CRP were measured at the time of presentation. Laboratory results at the time of presentation were recorded along with the final diagnosis. Receiver operating characteristic (ROC) curves were created using the data. Likelihood Ratios for ESR and CRP individually and in combination were calculated.

Results 259 patients were included in the study of whom 17 were considered to have an Orthopaedic infection. The ROC curves revealed the best results were obtained using cut off values of CRP>7 and ESR>12. The combination of a CRP>7 and an ESR>12 gave the best positive Likelihood Ratio at 6.26 (Likelihood ratio 5.34 (CRP>7) vs 2.57 (ESR>12)) For ruling out disease the combination of CRP≤7 and an ESR≤12 also outperformed either variable individually (negative likelihood ratio 0.09 (CRP≤7 and ESR≤12) vs 0.34 (CRP≤7) vs 0.18 (ESR≤12)).

Conclusion A raised CRP is a more specific indicator of orthopaedic infection than a raised ESR; however the ESR is more sensitive. Thus the Likelihood Ratios produced by the combination of the two results are superior to those of either measurement individually. The measurement of both CRP and ESR should be considered an important aid in the investigation of atraumatic limb pain.