Synovial Fluid White Cell Count in Knee Osteoarthritis: Association with Structural Findings and Treatment Response

OBJECTIVES: Osteoarthritis (OA) is a disease with a significant inflammatory component. The aim of this analysis was to determine the relationship between synovial fluid white cell count (SF WCC) and two parameters: disease severity and the reduction in knee pain after intra-articular steroid injection. METHODS: Subjects with painful knee OA were recruited for participation in an open label study of intra-articular steroid therapy. Information was obtained about knee pain using the KOOS questionnaire and a proportion of subjects had magnetic resonance imaging (MRI) performed. Prior to injection with methylprednisolone acetate (80mg), the index knee joint was aspirated and the fluid obtained forwarded for assessment of SF WCC. RESULTS: Information on SF WCC was available in 55 subjects. An increase in white cell count category (< 100, 101–250 and > 250–1,000 cells/mm(3)) was associated with an increase in synovial tissue volume (p = 0.028) and with other MRI-based measures of disease severity. Also, with each category increase in SF WCC there was a greater mean reduction in KOOS pain score after steroid injection; ≤100 cells/mm3 12.5 (SD 19.9) [referent], 101–250 cells/mm(3) 21.3 (SD 20.6) [β coefficient 0.279 p=0.049 ] and 251–1000 cells/mm(3) 29.3 (SD 15.2) [β coefficient 0.320 p=0.024]. CONCLUSION: Although within the ‘normal’ range, total synovial fluid white cell count appears to be a biomarker for MRI synovitis and may also predict response to anti-inflammatory treatment.