Identifying Inflammation in Knee Osteoarthritis: Relationship of Synovial Fluid White Blood Cell Count to Effusion‐Synovitis on Magnetic Resonance Imaging

Objective Inflammation is a potential pain generator and treatment target in knee osteoarthritis (OA). Inflammation can be detected on magnetic resonance imaging (MRI) and by synovial fluid white blood cell count (WBC). However, the performance characteristics of synovial fluid WBC for the detection of synovitis have not been established. This study was undertaken to determine the sensitivity and specificity of synovial fluid WBC in identifying inflammation in knee OA using MRI effusion‐synovitis as the gold standard. Methods We identified records of patients seen at an academic center with a diagnosis code for knee OA, a procedural code for knee aspiration, and a laboratory order for synovial fluid WBC in the same encounter, as well as an MRI within 12 months of the aspiration. MRIs were read for effusion‐synovitis using the MRI OA Knee Score (MOAKS). We dichotomized effusion‐synovitis as 1) none or small, or 2) medium or large. We calculated the sensitivity and specificity of synovial fluid WBC using MRI effusion‐synovitis (medium/large) as the gold standard. We used the Youden index to identify the best cut point. Results We included 75 patients. Mean ± SD age was 63 ± 12 years, and 69% were female. The synovial fluid WBC was higher in the medium/large effusion‐synovitis group (median 335 [interquartile range (IQR) 312]) than in the none/small group (median 194 [IQR 272]). The optimal cut point was 242, yielding a sensitivity of 71% (95% confidence interval [95% CI] 56–83%) and specificity of 63% (95% CI 41–81%). Conclusion The sensitivity and specificity of synovial fluid WBC in identifying effusion‐synovitis on MRI were limited. Further research is needed to better understand the association between MRI and effusion‐synovitis measured by synovial fluid and to determine which measure more strongly relates to synovial histopathology and patient outcomes.