Comparison of radioimmunoassay and enzyme-linked immunosorbent assay in measurement of antibodies to Neisseria meningitidis group A capsular polysaccharide.

Antibodies to meningococcal group A polysaccharide were determined by radioimmunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA) in serum samples from 16 adults vaccinated with bivalent meningococcal group A and C polysaccharide vaccine. The specific antibody levels in the serum samples were expressed as micrograms of antibody protein per milliliter of serum. For RIA the polysaccharide was radiolabeled extrinsically with 125I. Both native polysaccharide and polysaccharide labeled with 127I were used in ELISA. Because these antigens gave similar results, it can be concluded that the introduction of tyramine and iodine by the labeling procedure did not alter the antigenic activity of the polysaccharide. The reproducibility of RIA was clearly better than that of ELISA. The antibody levels detected by the methods were equal, which means that ELISA can be used satisfactorily to measure antibodies to meningococcal group A polysaccharide quantitatively. Some discrepant results were found due to an underestimation of immunoglobulin M antibodies in ELISA. This was shown by a correlation test in which a weakly significant negative correlation was found between the immunoglobulin M antibody level/immunoglobulin G antibody level ratio and the RIA antibody level/ELISA antibody level ratio.