The effect of the diffusion medium on quantification of human IgM by radial immunodiffusion

The results of quantification of human IgM by radial immunodiffusion (RID) are influenced by the charge of the diffusion medium and by the radio of IgM to other serum proteins. Agarose has almost no influence, while some heavily charged agars are unsuitable for RID, even after reduction of the IgM. Reference and test sera should not differ too widely in IgM levels. The interaction between charged agar and human IgM explains the subgroup phenomenon described earlier by Klein et al. (1973).