Hyponatremia in the Emergency Department: Could Biomarkers Help in Diagnosis and Treatment?

Objective: Hyponatremia is the most common electrolyte imbalance. The initial treatment decision is based on clinical evaluation of patient volume status but an accurate assessment is difficult, particularly differentiating mild hypovolemia from euvolemia. The aim of this study is to examine if biomarkers are valuable in the early determination of volume status and SIADH diagnosis. Methods: Blood samples were collected from an unselected patient population at entry to the Emergency Department. If the plasma sodium level (P-Na) was ≤125 mmol/L, the sample was frozen for further analysis. Mid-regional pro-atrial natriuretic peptide (MR-proANP), proadrenomedullin (MR-proADM), C-terminal prepro-vasopressin (copeptin), pro-endothelin-1 (proET-1) and N-terminal pro-brain natriuretic peptide (NT-proBNP) levels were analysed. A comprehensive assessment of volume status and underlying causes was made after discharge blinded for biomarker results. Results: A total of 81 patients were included. A well substa ntiated volemic state (hypo/eu/hypervolemia) was established in 72 patients (mean age 76 years, 65% women, median P-Na 119 mmol/L). A significant association was observed between MR-proANP levels and volemic state (p = 0.0001). Data was specifically analysed with respect to distinguishing hypo- from euvolemia (n = 59) using logistic regression. In a crude analysis, MR-proANP was significantly related to euvolemia (OR: 2.54 per SD of MR-proANP, 95% CI 1.32 - 4.86, p = 0.005) and remained so after the multivariate backward elimination model (OR: 2.45 per SD of MRproANP, 95% CI 1.22 - 4.91, p = 0.012.), whereas the other studied biomarkers were not. Copeptin levels were not associated with a diagnosis of SIADH. Conclusions: MR-proANP may be of value in early determination of volume status in hyponatremic patients.