[A monoclonal immunoglobulin M not visible on the profile of serum capillary electrophoresis].

We report a case of an invisible monoclonal IgM on the profile of the serum electrophoresis carried out by the Paragon CZE 2000 system. This case is about a 84 years old man followed for a Waldenström disease. The level of the monoclonal IgM given at the time of the preceding visit one year before was 10 g/L. The electrophoresis is reproduced in two other laboratories using respectively the Hydrasys and Capillarys techniques which have both shown the presence of the monoclonal peak. At this occasion the frequency of these anomalies of migration and the recommendations to avoid the erroneous interpretation of the electrophoresis are discussed. The vigilance and the competence of the biologist are essential to avoid this rare pitfall, induced by the new principle of the capillary electrophoresis.