ESR/CRP Ratio Identifies Reinfection Risk After Treatment of Chronic Prosthetic Joint Infection

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Prosthetic joint infection (PJI) is one of the most common reasons for revision of total joint arthroplasty and it is associated with significant morbidity for patients. The Musculoskeletal Infection Society has specified levels of erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) for evaluating PJI, but a recent systematic review in The Journal of Bone and Joint Surgery showed that the sensitivities and specificities of these biomarkers for PJI are low.

Meanwhile, interest has arisen in using the ESR/CRP ratio to predict infections in patients with non-orthopedic conditions. In one study ESR/CRP distinguished infection from flare in systemic lupus erythematosus, and in another it distinguished West Nile encephalitis from other viral central nervous system infections.

Stephen P. Maier, MD, Harvard Combined orthopaedic surgery resident, Young-Min Kwon, MD, PhD, program director of the Adult Reconstructive Surgery Fellowship Program and director of the Bioengineering Laboratory in the Department of Orthopaedic Surgery at Massachusetts General Hospital, and colleagues have found that preoperative ESR/CRP ratio predicts reinfection in patients whose PJI is treated with debridement, antibiotics and implant retention (DAIR). Their findings appear in The Journal of Arthroplasty.

Study Details

The retrospective study involved 179 consecutive patients who required revision total hip or total knee arthroplasty for PJI and underwent DAIR. They were categorized as having:

• Acute PJI—occurring >4 weeks after the initial surgery (12 patients developed reinfection, 41 did not)
• Acute hematogenous PJI—occurring >4 weeks postoperatively with a duration of symptoms <4 weeks (21 patients developed reinfection, 74 did not)
• Chronic PJI—occurring >4 weeks postoperatively (12 patients developed reinfection, 19 did not)

All patients had ESR and CRP measured before and after surgery and had at least 12 months of follow-up.

Differences in ESR/CRP

In the chronic infection group, but not the others, significant differences in ESR/CRP ratio were evident:

• Acute PJI—0.92 in patients who developed infection vs. 0.82 in those who did not (P = .70)
• Acute hematogenous PJI—0.87 vs. 1.06 (P = .56)
• Chronic PJI—2.33 vs. 1.23 (P = .04)

Discriminative Power

In subsequent analyses, the researchers used the area under the receiver operating characteristic curve (AUC) to estimate the discriminative power of ESR, CRP and ESR/CRP ratio. The larger the AUC (range, 0–1), the better the test was at discriminating between patients with and without PJI.

• Acute reinfection
  • ESR/CRP—AUC, 0.62; at cutoff of 0.43, sensitivity 83% and specificity 49%
  • ESR—AUC, 0.60
  • CRP—AUC, 0.44
• Acute hematogenous reinfection
  • ESR/CRP—AUC, 0.55; at cutpoint of 0.45, sensitivity 62% and specificity 53%
  • ESR—AUC, 0.44
  • CRP—AUC, 0.41
• Chronic reinfection
  • ESR/CRP—AUC, 0.80; at cutoff of 1.31, sensitivity 75% and specificity 84%
  • ESR—AUC, 0.48
  • CRP—AUC, 0.27

Applying the Findings to Practice

ESR and CRP are well established, reliable markers of inflammation that have long been routinely included in the diagnostic evaluation of periprosthetic hip and knee infection. They are also inexpensive to measure. Preoperative ESR/CRP ratio may be worth using to identify patients at increased risk of reinfection after they undergo DAIR for chronic PJI.

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