Rf-Absorbent for the removal of IgM rheumatoid factors from serum and plasma samples or cerebrospinal fluid (CSF) which are to be prepared for the detection of specific IgM antibodies in indirect immunoassays. Samples which are intended for the detection of pathogen-specific IgG antibodies are not to be treated with any Rf-Absorbent. RF-Absorbent to be used for any IgM ELISA test kit coded IB05xxx.