[Pseudohyponatremia in multiple myeloma (author's transl)].

In the present study frequency and mechanisms of alterations in serum electrolytes in multiple myeloma were investigated. Asymptomatic hyponatremia (up to 93 mmol/l) was present in 7 of 15 patients. Water of serum was significantly lower, viscosity of serum significantly raised as compared to healthy controls. A highly significant negative correlation between sodium concentration and viscosity was found. By measuring serum sodium concentration with viscosity independent dilution systems it could be proved that in each case the observed hyponatremia was caused by flaws in our routinely used viscosity dependent dilution method. The decrease in water of serum contributed only little to the development of hyponatremia. It is concluded that the decrease in serum sodium concentration in multiple myeloma is nothing else than a feigned hyponatremia which needs no therapy.