The 2013 Frank Stinchfield Award: Diagnosis of infection in the early postoperative period after total hip arthroplasty

Abstract

Background: Diagnosis of periprosthetic joint infection (PJI) can be difficult in the early postoperative period after total hip arthroplasty (THA) because normal cues from the physical examination often are unreliable, and serological markers commonly used for diagnosis are elevated from the recent surgery.

Questions/purposes: The purposes of this study were to determine the optimal cutoff values for erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), synovial fluid white blood cell (WBC) count, and differential for diagnosing PJI in the early postoperative period after primary THA.

Methods: We reviewed 6033 consecutive primary THAs and identified 73 patients (1.2%) who underwent reoperation for any reason within the first 6 weeks postoperatively. Thirty-six of these patients were infected according to modified Musculoskeletal Infection Society criteria. Mean values for the diagnostic tests were compared between groups and receiver operating characteristic curves generated along with an area under the curve (AUC) to determine test performance and optimal cutoff values to diagnose infection.

Results: The best test for the diagnosis of PJI was the synovial fluid WBC count (AUC = 98%; optimal cutoff value 12,800 cells/μL) followed by the CRP (AUC = 93%; optimal cutoff value 93 mg/L), and synovial fluid differential (AUC = 91%; optimal cutoff value 89% PMN). The mean ESR (infected = 69 mm/hr, not infected = 46 mm/hr), CRP (infected = 192 mg/L, not infected = 30 mg/L), synovial fluid WBC count (infected = 84,954 cells/μL, not infected = 2391 cells/μL), and differential (infected = 91% polymorphonuclear cells [PMN], not infected = 63% PMN) all were significantly higher in the infected group.

Conclusions: Optimal cutoff values for the diagnosis of PJI in the acute postoperative period were higher than those traditionally used for the diagnosis of chronic PJI. The serum CRP is an excellent screening test, whereas the synovial fluid WBC count is more specific.

Fig. 1A–D

(A) A ROC curve for the synovial WBC count is shown with an AUC of 98%. A cutoff value of 12,800 cells/μL demonstrates 89% sensitivity, 100% specificity, 100% PPV, 88% NPV, and 94% accuracy. (B) A ROC curve for %PMN is shown with an AUC of 91%. A cutoff value of 89% demonstrates 81% sensitivity, 90% specificity, 91% PPV, 79% NPV, and 85% accuracy. (C) A ROC curve for the serum CRP levels is shown with an AUC of 93%. A cutoff value of 93 mg/L demonstrates 88% sensitivity, 100% specificity, 100% PPV, 83% NPV, and 92% accuracy. (D) A ROC curve for the serum ESR is shown with an AUC of 73%. A cutoff value of 44 mm/hr demonstrates 92% sensitivity, 53% specificity, 76% PPV, 80% NPV, and 77% accuracy. PPV = positive predictive value; NPV = negative predictive value; CI = confidence interval.