Negative complement-fixation tests with rheumatoid factor positive sera. A simple method for selective removal of rheumatoid factor.

Sera from patients with rheumatoid arthritis regularly contain inhibitors of complement fixation (CF) to antigen-antibody complexes (Heimer and Levin, 1965). Removal of rheumatoid factor (RF) by absorption of sera with heat-aggregated human y-globulin resulted in a higher number of positives and a rise in CF antibody titre against antigens from herpes simplex, mumps, and morbilli viruses (Stanford, 1972). Penthinen, Wager, Pyrhbnen, Leinikki, and Teppo (1974) examined about 4,000 sera for CF with 25 viral antigens. They found that 1-4% were CF nonreactors though antibodies against some viral antigens could be shown by other methods. Most of the CF nonreactor sera showed high RF titres. The authors therefore recommend that a latex type RF test be carried out when a falsely low or negative CF is suspected. If a raised RF titre is found, the RF should be removed or destroyed before titrating CF antibodies. Absorption of sera with aggregated y-globulin is an expensive and time-consuming procedure. We have earlier shown that kaolin strongly adsorbs RF under conditions when antibodies to viral or bacterial antigens are not bound (Haukenes and Aasen, 1972). The efficiency of this method in unmasking CF antibodies in rheumatoid sera was therefore examined.