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Background: Hyponatremia and hypovolemia occur often after aneurysmal subarachnoid hemorrhage (SAH) and are associated with poor outcome. The authors investigated whether brain natriuretic peptide (BNP) is related to hypovolemia and hyponatremia after SAH and whether it can differentiate between hypovolemic and non-hypovolemic hyponatremia. Methods: In 58 SAH patients, the authors daily measured serum BNP and sodium concentrations, and circulating blood volume by means of pulse dye densitometry, during the initial 10 days. For each patient, mean BNP concentrations were calculated until occurrence of the following events: hyponatremia (Na <130 mmol/l), hypovolemia (blood volume <60 ml/kg), and severe hypovolemia (blood volume <50 ml/kg). The median day of onset of each event was calculated. In patients without an event, the authors calculated and used for comparison the mean BNP concentration until the median day of onset of the particular event. Odds Ratio's (OR) for high versus low mean BNP concentrations (dichotomized on median values per event) were calculated for the occurrence of each event and adjusted for relevant baseline characteristics. Results: Patients with BNP above median more often had severe hypovolemia (adjusted OR 4.2, 95% confidence interval, CI 1.2-15.0) and showed a trend toward hyponatremia (adjusted OR 3.3, 95% CI 0.7-9.2). In the 12 hyponatremic patients, BNP could not differentiate between hypovolemic and non-hypovolemic hyponatremia. Conclusions: High BNP concentrations are related to the occurrence of severe hypovolemia and possibly hyponatremia. These data do not support a role for BNP measurements to differentiate between hypovolemic and non-hypovolemic hyponatremia in SAH patients. [ABSTRACT FROM AUTHOR]