A modified Farr technique for the detection of antibody in the presence of antigen-antibody complexes.

Antibody bound in an antigen-antibody complex was barely detectable by the standard Farr technique using 10 microgram/ml of radiolabeled antigen. When the Farr test was repeated using larger concentrations of antigen the ABC measurements increased several-fold. In separate experiments complexes were dissociated with low pH buffer. The radiolabeled antigen was then added and the mixture returned to neutral pH before the Farr assay was performed. Pretreatment of the complexes with citrate-buffered saline pH 3.1 enhanced antibody measurements 1.5-3.6-fold. Pretreatment with glycine-buffered saline pH 2.2 enhanced complexed antibody measurements 2.1-4.8-fold. The low pH buffers did not affect ABC values on free antibody. Antibody bound in immune complexes can be detected with a modified Farr technique using a high concentration of antigen and/or by pretreatment of the complexes with low pH buffers before the addition of radiolabeled antigen.