Lack of laboratory assessment of severe hyponatraemia is associated with detrimental clinical outcomes in hospitalised patients.

OBJECTIVE: Increased mortality with severe hyponatraemia is well known. What is less clear is the mortality risk according to the pattern of the developing hyponatraemia and whether this may be affected by the intervention of the clinician.
METHODS: From our laboratory database, we retrospectively collected data of a 12-month period of adult patients with severe hyponatraemia (< or = 120 mmol/l). One hundred and thirteen patients were identified. Normonatraemic controls (n = 113) were identified by plasma sodium of 135 mmol/l over the same period, and whose nadir during hospitalisation was > or = 130 mmol/l. Results are mean +/- SD unless stated otherwise. Duration of hospitalisation and clinical outcomes was confirmed from hospital records.
RESULTS: The mean nadir plasma sodium of the hyponatraemic group was 116.0 +/- 4.4 mmol/l and 134.0 +/- 2.8 mmol/l in controls. Although the hyponatraemic patients were younger than controls (65.8 +/- 18.4 vs. 72.3 +/- 14.9 years; p = 0.004), they had higher mortality (24 vs. 7, p = 0.002) and longer hospitalisation than controls: median (IQR), 12 (7-22) vs. 7 (3-16.5) days (p < 0.001). A total of 55 patients developed severe hyponatraemia following admission. This subgroup comprised a higher proportion of surgical patients (23.6% vs. 1.7%, p < 0.001) than those with severe hyponatraemia on admission. Furthermore, both mortality (n = 17 vs. n = 7; p = 0.02) and duration of hospitalisation, median 19 days (IQR 10-35) vs. 9.5 (5-15) days (p < 0.001), were greater. Failure to measure plasma and urinary osmolalities was associated with increased mortality.
CONCLUSIONS: Severe hyponatraemia is associated with prolonged admission and increased mortality compared with normonatraemic patients. Progressive hyponatraemia following admission incurs a higher risk of death. This may represent illness-severity, inappropriate management or inadequate investigation.