RESUMO
Pseudo-hyponatremia is a rare condition characterized by a decrease in plasma dodium concentration associated with a normal or increase effective plasma osmolarity. We present the case of a 62 year old woman with multiple myeloma (IgG) who had a plasma sodium of 84,5 mEq/l (as measured with flame photometry) and an anion gap of - 8,8 mEq/l. However, when determined with an ion-selective electrolde, plasma sodium was 135 mEq/l. The patient was treated with an ion-selective electrode, plasma sodium was 135 mEq/l. the patient was treated with chemotherapy and did not receive sodium. One week later, plasma sodium - again measured by flame photometry - was 134 mEq/l and the anion gap 4.0 mEq/l. The pathophysiological and clinical aspects of pseudo-hyponatremia and the anion gap are discussed