Demonstration of rubella-specific IgM by serum ultracentrifugation on sucrose gradients: comparison of a vertical and a swinging bucket rotor.

The performances of a vertical and a swinging bucket rotor in the diagnosis of rubella-specific IgM were compared using two sets of predominantly low titre rubella IgM positive sera. Applying a number of subtle criteria, the vertical rotor was shown to separate IgM and IgG less well than the swinging bucket rotor. If the vertical rotor was loaded with pretreated sera with IgG content decreased severalfold, its performance was similar to that of the swinging bucket rotor with native serum. The findings are discussed from the aspect of the main indication of rubella IgM testing, and it was concluded that for the majority of cases the vertical rotor would not present an alternative to the swinging bucket rotor.