Hyponatraemia during an emergency medical admission as a marker of illness severity & case complexity.

BACKGROUND: Altered sodium balance at time of an emergency medical admission adversely impacts on outcome; whether hyponatraemia is independently associated with outcomes or a surrogate of acute illness severity has been debated.

METHODS: All emergency medical admissions between 2002 and 2017 were studied and a risk score calculated. We compared univarate deciles of admission sodium using a multivariable model, adjusting for risk score.

RESULTS: There were 106,586 admissions in 54,928 patients. Patients with lower sodium at admission were older at 66.7 years (IQR 46.7-79.5) compared with 63.3 years (IQR 42.9-78.2) with a longer length of stay (LOS) of 6.8 days (IQR 3.0-14.7) versus 4.9 days (IQR 1.8-10.9). They had a higher 30-day in-hospital mortality at 6.4% vs 4.4% (p < 0.001). Admission sodium predicted survival - OR 0.89 (95%CI 0.88-0.90). We adjusted the model with a Risk Score that is predictive and exponentially related to 30-day in-hospital mortality. When adjusted for Risk Score, the admission sodium value was less predictive - OR 0.95 (95%CI 0.92-0.97). The cumulative percentages within the lowest five deciles fell from 63.3% between 2002 and 2009 to 48.1% from 2010 to 2017. The slope of the prediction line relating admission sodium to mortality did not change over time but a lower mortality rate was predicted at any given sodium level.

CONCLUSION: Hyponatraemia at the time of an emergency medical admission is predictive and probably a marker of Acuity Illness Severity and Case Complexity. Both the frequency of abnormality in admission sodium and mortality have fallen over time.