Critical Differences between Pneumococcal Polysaccharide Enzyme-Linked Immunosorbent Assays with and without 22F Inhibition at Low Antibody Concentrations in Pediatric Sera

A comparative study was conducted between two laboratories in order to evaluate the differences between two enzyme-linked immunosorbent assay (ELISA) techniques for the detection of pneumococcal anti-capsular polysaccharide antibodies. One laboratory used an assay including heterologous 22F polysaccharide inhibition, and the other laboratory employed a non-22F reference assay. After conjugate immunization, 30 pediatric post-primary immunization sera with antipolysaccharide concentrations ranging from <0.05 to 15 mug/ml were analyzed. Aggregate reverse cumulative distribution curves combining concentrations of antibodies against serotypes 4, 6B, 9V, 14, 18C, 19F, and 23F revealed similar results for both methods at antibody levels of >1 microg/ml. However, at antibody levels of <1 microg/ml, the distribution curve measured with the 22F inhibition ELISA shifted toward lower levels. This observation suggests that the 22F inhibition assay is more specific at low antibody concentrations, which was confirmed by heterologous polysaccharide inhibition experiments. Translation of low antibody levels suggested that the proposed threshold concentration of 0.35 mug/ml determined with the non-22F ELISA corresponded to a concentration of 0.20 mug/ml with the 22F inhibition ELISA. Pneumococcal antipolysaccharide ELISA including 22F inhibition can be recommended as a reference method.