A MODIFIED FARR ASSAY IS MORE SPECIFIC THAN ELISA FOR MEASURING ANTIBODIES TO STREPTOCOCCUS-PNEUMONIAE CAPSULAR POLYSACCHARIDES

In an attempt to develop an assay specific for antibody to capsular po lysaccharide (PS) of Streptococcus pneumoniae, the ability of ELISA an d Farr (radioactive antigen-binding) assay for antibodies to 6B and 19 A PS to be affected by antibodies to C-polysaccharide (C-PS) was compa red. Preabsorption with C-PS reduced values obtained by ELISA for anti -6B antibody by >3-fold in 5 of 10 preimmune and 7 of 26 postimmune se ra. In contrast, absorption reduced values by >3-fold in 0 of 36 sampl es studied with the Farr assay, Similar results were observed when the absorption was done with CSR-SCS2 S. pneumoniae. Furthermore, when an ti-19A antibody levels were examined, preabsorption with R36a S. pneum oniae reduced ELISA values by 3-fold in 7 of 22 samples, whereas no sa mples had 3-fold reduction by Farr assay. Thus, the Farr assay for cap sular PS is less affected than the ELISA by anti-C-PS antibody.