Are Nucleated Cell Counts Useful in the Diagnosis of Infection in Periprosthetic Fracture?

BACKGROUND: Evaluating for the possibility of prosthetic joint infection in the setting of periprosthetic fracture is important because it determines the course of treatment. However, fracture-related inflammation can make investigations used in the diagnosis of infection less reliable. QUESTIONS/PURPOSES: The purpose of our study was to evaluate synovial fluid nucleated cell counts as a diagnostic test for deep prosthetic infection in patients with periprosthetic fractures around hip and knee arthroplasties. Specifically, we wished to determine the test's properties (sensitivity, specificity, positive predictive value [PPV], and negative predictive value [NPV]) using threshold levels of nucleated cell counts as they are otherwise used in the diagnosis of periprosthetic infection.
METHODS: Billing codes were used to identify all cases of revision total hip arthroplasty (THA), revision total knee arthroplasty (TKA), open reduction and internal fixation (ORIF) of the femur, and ORIF of the tibia at our institution between 2005 and 2013. A total of 2537 charts were identified and reviewed to reveal 269 patients with 269 periprosthetic fractures about a THA or TKA (10.6% of charts reviewed). Of these, 27 fractures in 27 patients (10% of the periprosthetic fractures identified) underwent aspiration of their total joint arthroplasty to rule out infection before surgical intervention. The decision to aspirate was made by the treating surgeon based on clinical suspicion of infection from the patient history, physical examination, and radiographic findings. Nucleated cell counts from joint aspirates were recorded for all 27 patients. Synovial fluid culture results were then used to calculate the sensitivity, specificity, PPV, and NPV of an elevated nucleated cell count in the diagnosis of infection.
RESULTS: The specificity, sensitivity, PPV, and NPV of an elevated nucleated cell count in the diagnosis of infection were 64% (95% confidence interval [CI, 34.94-75.57]), 100% (95% CI, 19.29-100), 18% (95% CI, 2.37-45.46), and 100% (95% CI, 76.66-100), respectively. Eleven of 27 patients (41%) with joint aspirates had elevated nucleated cell counts. Only two of the 11 patients (18%) with elevated nucleated cell counts had positive synovial fluid cultures. None of the patients with normal nucleated cell counts had positive synovial fluid cultures.
CONCLUSIONS: Although quite common, an elevated nucleated cell count has moderate specificity and poor PPV in the diagnosis of infection in the setting of periprosthetic fracture. LEVEL OF EVIDENCE: Level IV, therapeutic study.