When diagnosing loose components after total knee arthroplasty (TKA), orthopaedists often turn to synovial-fluid analysis to help them differentiate between septic and aseptic causes. But how useful are such analyses clinically? Although that was not the primary research question in the October 7, 2015 JBJS article by Chalmers et al., their findings shed light on the utility of synovial white blood-cell (WBC) counts in differentiating aseptic from septic loosening.
When the authors compared synovial-fluid characteristics among patients with periprosthetic infections to those among patients with various modes of aseptic failure (including extensor mechanism failure, component malposition, polyethylene wear, and periprosthetic fracture), they concluded that “to maximize the diagnostic accuracy of synovial aspiration, different [WBC] cutoffs may need to be employed depending on the clinical scenario and the alternative diagnosis being considered.”
For example, Chalmers et al. found that if there was radiographic evidence of component malposition at the same time a surgeon suspected periprosthetic infection, the optimal WBC cutoff would be 2104 cells/µL (sensitivity=95%; specificity=100%). But if a patient presented with a periprosthetic fracture and the surgeon suspected that the fracture may have occurred as a consequence of septic loosening, the optimal WBC cutoff would be 4697 cells/µL (sensitivity=89%; specificity=100%). Meanwhile, the current AAOS Clinical Practice Guideline cutoff for chronic periprosthetic infection is 1700 cells/µL.
The variability in these findings led the authors to conclude that “the optimal diagnosis of periprosthetic infection on the basis of synovial aspiration results may need to utilize different cutoff values depending on the alternative mode of failure being considered.”