Furthermore, we have also related serum concentrations of COMP-C3b complexes to clinical and biochemical markers commonly used to monitor disease activity, as well as studied changes of COMP-C3b levels over a 3-month period in relation to treatment of RA, PsA, or AS with TNF inhibitors to elucidate how changes relate to modulation of disease activity. Here, COMP is linked to rheumatoid arthritis.