Pseudohyponatremia is an uncommonly encountered laboratory abnormality defined by a serum sodium concentration of less than 135 mEq/L in the setting of a normal serum osmolality (280 to 300 mOsm/kg). Conversely, true hyponatremia is associated with low serum osmolality and should prompt evaluation for the presence of an additional abnormal solute that may be affecting the laboratory assessment. The finding of pseudohyponatremia is an artifact that results from the way a blood sample is processed for serum sodium measurement.

Some sources cite the presence of osmotically-active solutes, such as mannitol or hyperglycemia, as an additional etiology of pseudohyponatremia. It is preferable to identify pseudohyponatremia based on false laboratory analysis, in which the laboratory result does not match the actual sodium level. Due to the hyperosmolality and resultant fluid shifts invoked by the presence of osmotically-active solutes, the serum sodium, as reported by laboratory assessment in these cases, is truly low. For this reason, one should not include hyperosmolar hyponatremia as a potential classification of pseudohyponatremia.