Background: Hyponatremia (serum sodium < 135 mEq/L) is the most common electrolyte abnormality in hospital and has impact on patient morbidity and mortality. The accuracy of volume status assessment is a major problem for the treatment planning especially to discriminate mild hypovolemic from euvolemic patients.
Objective: To examine the relationship between plasma N-Terminal-pro-Brain Natriuretic Peptide (NT-pro-BNP) level and extracellular water (ECW) status during the treatment of hyponatremia, as well as the cut-off value of plasma NT-pro-BNP in the differential diagnosis of volume status in hypovolemic vs. euvolemic hyponatremic patients.
Material and Method: Hyponatremic patients without clinical hypervolemia in Rajavithi Hospital were divided into the hypovolemic group and the euvolemic group according to ECW volume determined by bioimpedance analysis (BIA). Serum sodium, plasma NT-pro-BNP and ECW were assessed at the beginning, at the half correction of hyponatremia and at the end of treatment.
Results: Of the 26 patients, 18 (69.2%) were hypovolemic and 8 (30.8%) were euvolemic. Before treatment, NT-pro-BNP levels of the patients with hypovolemia was significantly lower than the patients with euvolemia [median (min, max)] (pg/mL) of hypovolemic vs. euvolemic group [114 (21, 6,803) vs. 1,509 (538, 8,541)] respectively (p < 0.001) and NT-pro-BNP levels change in the similar direction as ECW volume during the treatment. The best cut-off value of plasma NT-pro-BNP level to distinguish hypovolemic from euvolemic hyponatremia was 518 pg/ml with the sensitivity of 94.4% and the specificity of 100%.
Conclusion: Plasma NT-pro-BNP levels provide objective information with respection to volume status in hyponatremia patients and can be used in clinical diagnosis of hypovolemic vs. euvolemic hyponatremic hyponatremia.

Keywords: N-terminal-pro brain natriuretic peptide, Volume status, Hyponatremia