Brief Report: Synovial Fluid White Blood Cell Count in Knee Osteoarthritis: Association With Structural Findings and Treatment Response.

Objective: Osteoarthritis (OA) is a disease with a significant inflammatory component. The aim of this analysis was to determine the relationship between synovial fluid (SF) white blood cell (WBC) count and 2 parameters: disease severity and the reduction in knee pain after intraarticular (IA) steroid injection.

Methods: Subjects with painful knee OA were recruited for participation in an open-label study of IA steroid therapy. Information was obtained about knee pain using the Knee Injury and Osteoarthritis Outcome Score (KOOS) questionnaire, and a proportion of subjects underwent magnetic resonance imaging (MRI). Prior to injection with 80 mg methylprednisolone acetate, the index knee joint was aspirated and the fluid obtained was forwarded for assessment of SF WBC count.

Results: Information on SF WBC count was available for 55 subjects. An increase in WBC count category (≤100, 101-250, and 251-1,000 cells/mm ) was associated with an increase in synovial tissue volume (P = 0.028) and with other MRI-based measures of disease severity. Also, with each increase in SF WBC count category, there was a greater reduction in KOOS score after steroid injection (for WBC count of ≤100 cells/mm [referent], mean ± SD 12.5 ± 15.2; for WBC count of 101-250 cells/mm , mean ± SD 21.3 ± 20.6 [β coefficient 0.279, P = 0.049]; for WBC count of 251-1,000 cells/mm , mean ± SD 29.3 ± 15.2 [β coefficient 0.320, P = 0.024]).

Conclusion: Although all participants in the analysis had SF WBC counts within the "normal" range, total SF WBC count appears to be a biomarker for synovitis on MRI and may also predict response to antiinflammatory treatment.