Evaluation of NT-ProBNP as a marker of the volume status of neurosurgical patients developing hyponatremia and natriuresis: A pilot study

Objective: Post-operative hyponatremia (serum sodium <130 mEq/L) contributes to morbidity and prolongs the hospital stay of patients undergoing neurosurgical procedures. Syndrome of inappropriate anti-diuretic hormone secretion (SIADH) and cerebral salt wasting (CSW) commonly occur in the post-operative setting. While patients with SIADH are either euvolemic or hypervolemic, patients with CSW are always hypovolemic. The treatment of these two conditions is radically different. Patients with SIADH need fluid restriction, while patients with CSW need fluid replacement. As current diagnostic methods do not clearly distinguish between SIADH and CSW, we looked at N-terminal prohormone of brain natriuretic peptide (NT-proBNP) and uric acid as biochemical markers for estimating the volume status of patients developing hyponatremia in the postoperative period. Materials and Methods: In this study, we used a cohort design and carried it out in two phases over a period of 30 months (August 2011-February 2014). Thirty-one patients with hyponatremia were recruited into the study. In Phase1, 10 patients were diagnosed to have either SIADH or CSW based on their central venous pressure (CVP). In all of them, blood for NT-proBNP was collected prior to starting treatment. At a later stage, the NT-proBNP results were compared with central venous pressure (CVP) and the clinical diagnosis. Patients diagnosed to have SIADH (CVP >5cm) had NT-proBNP levels <125pg/ml and those with a diagnosis of CSW (CVP <5cm) had NT-proBNP levels >125pg/ml. In Phase2, 21 patients were categorized and treated according to their NT-proBNP levels. Those with NT-proBNP levels <125 pg/ml were treated as SIADH, and those with NT-proBNP levels >125 pg/ml were treated as CSW. Results: In Phase 1, NT-proBNP could detect hypovolemia in patients with CSW with 100% sensitivity and 66.7% specificity (P < 0.07). In Phase 2, NT-proBNP could detect hypovolemia in patients with CSW with 90% sensitivity and 100% specificity (P < 0.001). Combining the results of Phase 1 and Phase 2, NT-proBNP could diagnose CSW with 87.50% sensitivity and 93.33% specificity (P < 0.001). The positive predictive value was 93.33% and the negative predictive value was 87.50%. Conclusion: NT Pro-BNP is a quick and convenient assay to differentiate SIADH and CSW. We need a larger sample size to correctly characterize the cut off value. Uric acid cannot be used to distinguish between SIADH and CSW.