Study points to value of neutrophil percentage over WBC count to predict PJI in MoM total hip arthroplasty

CHICAGO — The results of a retrospective review of revised metal-on-metal (MoM) total hip arthroplasties caution against the use of synovial fluid white blood cell (WBC) counts to diagnose periprosthetic joint infection (PJI) and instead highlight the accuracy of neutrophil percentages for this purpose.

“Synovial fluid white blood cell counts in metal-on-metal hip arthroplasty can be extremely variable and should be interpreted with caution in the diagnosis of infection. In contrast, synovial fluid neutrophil percentage demonstrates both excellent sensitivity and specificity in the diagnosis of periprosthetic joint infection,” Cody Wyles, BS, said during his presentation here at the American Academy of Orthopaedic Surgeons Annual Meeting. “In this specific patient population, it can add to the diagnostic confidence in the appropriate clinical context,” Wyles said.

Wyles and Robert T. Trousdale, MD, senior author and Orthopedics Today Editorial Board member, identified 38 patients (39 hips) who underwent revision of a failed MoM total hip arthroplasty (THA) at the Mayo Clinic, in Rochester, Minn., between Aug. 2006 and March 2012 and also had a preoperative synovial fluid aspiration within in 1 month of their procedure. The patients’ mean age was 60 years at the time of revision surgery and there were 23 women in the cohort.

The main reasons for revision included pain, adverse reactions to metal debris and infection. An analysis of synovial fluid aspirations and intraoperative specimens revealed negative cultures in 35 hips and positive cultures in four hips.

The investigators found a mean synovial fluid WBC count of 18,384 cells per microliter for the uninfected cases and 150,743 cells per microliter for the infected group. There were mean neutrophil percentages of 41% in the uninfected group and 97% in the infected cohort, Wyles said.

Using a WBC count of > 3,000 cells per microliter to predict PJI, the researchers discovered that synovial fluid WBC count had 100% sensitivity, but 57% specificity.

“Neutrophil percentage, on the other hand, had a sensitivity of 100% and a high specificity of 97%,” Wyles said. Using an erythrocyte sedimentation rate (ESR) of > 22 m/hr and C-reactive protein (CRP) level of greater than 8 mg/L, both showed 75% sensitivity and 67.6% specificity.

“ESR and CRP both demonstrated mediocre sensitivity and specificity,” according to Wyles.

Reference:

Wyles C. Paper #68. Presented at: American Academy of Orthopaedic Surgeons Annual Meeting; March 19-23, 2013; Chicago.

Disclosures: Wyles has no relevant financial disclosures.