Relationship between serum viscosity and intravascular IgA polymer concentration in IgA myeloma

The relative serum viscosity (RSV) at 37 degrees C was measured at presentation in all 179 patients with IgA myeloma admitted to the IIIrd and IVth British Medical Research Council trials until January 1981. An elevated relative serum viscosity (greater than 1.7) was noted in 83%. Hyperviscosity, as defined by a relative serum viscosity greater than 5, was a comparatively rare presenting feature (5.0%). The relationship between relative serum viscosity and IgA polymer equivalent concentration was examined in 33 of the 179 patients, selected to cover a wide range of serum viscosity. A significant correlation was observed between the two parameters (rs=0.7928, P less than 0.001). The sera from two patients with marked serum hyperviscosity (RSV=9.51 and 9.76) were clearly distinct from this relationship. Protein aggregation appeared to be a major determinant in the development of serum hyperviscosity in these two patients.