Acute or chronic periprosthetic joint infection? Using the ESR ∕ CRP ratio to aid in determining the acuity of periprosthetic joint infections.

Introduction: The gold standard for determining the duration of periprosthetic joint infection (PJI) is a thorough history. Currently, there are no well-defined objective criteria to determine the duration of PJI, and little evidence exists regarding the ratio between ESR (mm/h) and CRP (mg/L) in joint arthroplasty. This study suggests the ESR  $/$  CRP ratio will help differentiate acute from chronic PJI. Methods: Retrospective review of patients with PJI was performed. Inclusion criteria: patients $>18$  years old who underwent surgical revision for PJI and had documented ESR and CRP values. Subjects were divided into two groups: PJI for greater (chronic) or less than (acute) 4 weeks and the ESR  $/$  CRP ratio was compared between them. Receiver-operating characteristic (ROC) curves were evaluated to determine the utility of the ESR  $/$  CRP ratio in characterizing the duration of PJI. Results: 147 patients were included in the study (81 acute and 66 chronic). The mean ESR  $/$  CRP ratio in acute patients was 0.48 compared to 2.87 in chronic patients ( $p<0.001$ ). The ESR  $/$  CRP ROC curve demonstrated an excellent area under the curve (AUC) of 0.899. The ideal cutoff value was 0.96 for ESR  $/$  CRP to predict a chronic ( $>0.96$ ) vs. acute ( $<0.96$ ) PJI. The sensitivity at this value was 0.74 (95 % CI 0.62-0.83) and the specificity was 0.90 (95 % CI 0.81-0.94). Conclusions: The ESR  $/$  CRP ratio may help determine the duration of PJI in uncertain cases. This metric may give arthroplasty surgeons more confidence in defining the duration of the PJI and therefore aid in treatment selection.