Establishment of a quantitative ELISA assay recommended by WHO for the detection of human IgG antibodies specific for Streptococcus pneumoniae capsular polysaccharides （ Pn PS ELISA）
Objective To establish a standardized quantitative enzyme-linked immunosorbent as- say （ELISA） recommended by WHO for the detection of human IgG antibodies specific for Streptococcus pneumoniae capsular polysaccharides （Pn PS ELISA）. Methods According to the WHO recommended standard Pn PS ELISA protocol, capsular polysaccharide concentrations of 13 serotypes （1, 3, 4, 5, 6A, 6B, 7F, 9V, 14, 18C, 19A, 19F, 23F） of Streptococcus pneumonia for coating were optimized; the ELISA plates and AP conjugated goat anti-human IgG antibody for the detection were experimentally selected. Using the established assay parameters assured by testing quality control standards （QCs） provided by WHO refer- ence laboratory, a panel of 16 LIBP （ Lanzhou Institute of Biological Products Co. Ltd. ） QCs were measured for comparison analysis between WHO reference laboratory and LIBP laboratory. In the meantime, the range of IgG concentrations of an internal QC panel 907 for 13 serotypes of pneumococcal capsular polysaccharide were established for routine QC work in LIBP laboratory, and inter-assay precision within LIBP laboratory was assessed as well. Results The standardized Pn PS ELISA assay established in LIBP laboratory met the criteria required by WHO. There was a high correlation between the data collected by WHO reference labora- tory and LIBP laboratory （ slope = 0.94, coefficient of correlation r = 0. 97, P〈0.05 ）. Eighty one percent ofIgG concentrations measured by LIBP laboratory were within the range of ＋40% of those measured by WHO reference laboratory, which met the criteria of 75% data falling within the ambit of ＋-40% of assigned values commonly used for comparison. The ranges of IgG concentration in LIBP QC 907 for 13 serotypes had been established. The inter-assay precision in LIBP laboratory was high with coefficiency of variation （CV） less than 30%. Conclusion LIBP laboratory has successfully established the standardized ELISA recommended by WHO for quantitative detection of human IgG antibodies against pneumococcal capsular polysaccharides （ Pn PS ELISA）. The range of IgG concentrations in LIBP QC 907 for 13 seretypes of pneumococcal capsular polysaccharide are also established for routine quality-control practice. 
 
Key words: 
Streptococus pneumonia;  IgG concentration against capsular polysaccharide ;  Enzyme- linked immunosorbent assay （ELISA）