Concentrations of S100A8 and S100A9 proteins in the serum and synovial fluid (SF) of rheumatoid arthritis (RA) and osteoarthritis (OA). Paired serum and SF samples were prepared from patients with RA and patients with OA. Concentrations of the S100A8 and S100A9 homodimer and the S100A8/A9 heterodimer were measured in duplicate by enzyme-linked immunosorbent assay. Values are the mean ± standard error of the mean. n, number of samples tested.