Plasma volume expansion and increased serum viscosity in myeloma and macroglobulinaemia.

Twenty-three simultaneous determinations of relative serum viscosity and plasma volume were made on a random selection of fifteen patients with IgG or IgA myeloma, or Waldenström's macroglobulinaemia. There was a highly significant correlation (P =0·001) between the presence of an elevated relative serum viscosity and an increased plasma volume. This applied equally to type IgA, IgG or IgM. Eleven patients had a relative viscosity >2·0 (normal 1·4–1·8), and expansion of plasma volume was not found when the relative viscosity was <2·0. The cause of the plasma volume expansion cannot be a simple oncotic effect, and is still not clear.