Reorienting gradient centrifugation in the determination of rubella specific IgM.

The laboratory diagnosis of acute or congenital rubella frequently rests on the presence of rubella specific IgM in serum.1'2 Separation of serum IgM from IgG by sucrose density gradient centrifugation followed by the demonstration ofhaemagglutinationinhibiting activity in the IgM containing fractions is widely used for this.' With swing-out rotors centrifugation times of 16 hours or more are necessary. Use of reorienting sucrose gradients in a fixed-angle, vertical rotor allows this time to be considerably reduced. In this type of rotor, a sucrose gradient is gently accelerated, during which time it reorientates through 900 so that its length is the diameter of the centrifuge tube. Molecules then have a shorter distance to travel before adequate separation is achieved. The gradient returns to its original position when the rotor is decelerated. This report describes the use of such a rotor in the determination of rubella specific IgM.