COMP, an inflammation biomarker in knee OA. Measurement of serum COMP levels in 88 OA patients examined by ultrasound to assess severity of OA disease and presence of synovitis showed serum COMP levels ranging from 52 to 66.5 ng/mL and correlated with the clinical severity of OA and the involvement of synovitis in knee OA. COMP thus was a biomarker of the severity of inflammation in knee OA. Here, COMP is linked to synovitis.