Pseudohyponatremia is caused by an increased serum protein or lipid concentration
producing a "space-occupying lesion" in serum water. Its presence and magnitude must
be assessed in hyponatremic patients with, for example, paraproteinemia or hyperlipemic
diabetic coma. In the absence of a direct-reading ion-selective electrode system,
a method for measuring the water content of serum is required. We describe two rapid
methods for measuring the diffusible water of serum: osmometry before and after dilution
and chloride measurement before and after ultrafiltration. Either of these methods
allows the true sodium status of a patient's serum to be determined.