Synovial Fluid Analyses Often Faulty for Patients With Failed MoM Bearings

In the diagnosis of periprosthetic joint infection for patients with failed metal-on-metal bearings or corrosion, synovial fluid analyses frequently are false-positive and only should be relied upon if manual counts or automated differentials are done, according to recent data presented last week at the Current Concepts in Joint Replacement Winter Meeting in Orlando, Fla.

In the Orthopaedic Research and Education Foundation / Current Concepts in Joint Replacement Clinical Award Paper presented, Michael B. Cross, M.D., said a third of the cases in his retrospective study had inaccurate automated results, which can lead to unacceptable high false-positive rates.

“We recommend to alert the lab technician when you are sending the samples in these cases and ask for a manual count if the automated count is questionable,” Cross said. “If nothing else, it alerts the technician to examine the specimen closely, and automated results are ultimately more reliable if the differential can be performed.”

He also recommended performing these aspirations preoperatively so the culture results can act as a guide if the aspiration is not accurate.

Cross and former colleagues at Rush University Medical Center inChicago, Ill., analyzed the utility of commonly used tests for the diagnosis of periprosthetic joint infection in patients who had failed metal-on-metal (MoM) bearings or corrosion reactions. Overall, they evaluated 150 hips, of which 92 were MoM total hips, 10 were MoM hip resurfacing, 30 were non-MoM bearings with corrosion and nine were full-thickness-bearing surface wear with metallosis.

Cross said 19 hips met the Musculoskeletal Infection Society criteria for infection, and there were 131 aseptic loosenings. Mean age at surgery was 59.2 years for the 78 men and 72 women in the study. Mean time to revision was 56.3 months.

Automated synovial fluid white blood cell (WBC) counts were found to be inaccurate secondary to cellular debris in 47 patients, according to Cross. A WBC count was initially reported in 35 hips, of which 11 hips were false-positive.

In a comparison of the groups’ mean laboratory values and test performances, the researchers found infected patients showed significantly higher mean serum erythrocyte sedimentation rates, C-reactive protein levels, synovial fluid WBC counts and differentials. Diagnostic performance of the synovial fluid WBC count and differential improved with fewer false-positives after inaccurate samples were excluded, according to Cross.

Overall, Cross said 66.6 percent of the automated cell counts were accurate, and diagnostic utility was similar among the groups.