C3 and rheumatoid arthritis: Accordingly, to demonstrate that the C3f assay is mainly detecting the C3f fragment within C3b molecule in the RA serum, 5 of the positive RA serum samples from Fig 2A above were put through 10kDa and 3kDa cut-off filter columns (Bio-Rad, UK) prior to analysis by the C3f sandwich ELISA.