Highly sensitive biotin-avidin sandwich ELISA for the rapid detection of pneumococcal capsular polysaccharide antigens

The immunological detection of soluble pneumococcal polysaccharide antigens in pathological products is of importance in the direct diagnosis of meningitis or pulmonary infections. We have developed a double antibody sandwich ELISA method using a biotin-avidin system using antibodies constituted with a mixture of IgGs from pooled and/or monospecific antipneumococcal sera provided by the Danish Statens Seruminstitut. The sensitivity of this rapid ELISA method was optimized with purified capsular polysaccharides of the 24 pneumococcal serotypes. With incubation steps of 30 min at 37°C for the antigens and the conjugates, the detection limit was close to 1 ng/ml for 75% of the purified polysaccharides. A retrospective study of 46 CSF samples established the validity of the assay. This type of modified ELISA system represents a specific, sensitive and rapid procedure for the potential detection of capsular soluble antigens of all pneumococcal serotypes.