White blood cell counts and differential in synovial fluid of aseptically failed total knee arthroplasty

A cell count and differential was obtained on synovial fluid samples from 79 total knee arthroplasties undergoing revision for aseptic failure over a 5-year period. The mean white blood cell count (WBC) was 782/mL (range, 11–7, 200/mL). The mean percentage of monocyte cells was 87%, and the mean polymorphonuclear leukocyte (PML) cell count was 13%. Eight patients (10.4%) had leukocyte cell counts >2, 000. Five knees in 4 patients with rheumatoid arthritis had PML counts >50%. There was a single postoperative infection. Inpatients with osteoarthritis as a primary diagnosis, a synovial WBC of <2, 000 and a differential with <50% PMLs had a 98% negative predictive value for the absence of infection.