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How we define hyponatraemia?

Sir, We read with interest the article ‘The diagnosis and management of inpatient hyponatraemia and SIADH’ of Grant et al. (1). Hyponatraemia has been variably defined as a serum sodium concentration < 135 mmol/L (2), < 136 mmol/L (3) or even < 138 mmol/L (4), whereas severe hyponatraemia is conventionally defined as a serum sodium concentration < 125 mmol/ L, as for widespread consensus (3). Interestingly, a precise definition of hyponatraemia has not been provided in the article of Grant et al. whereas this condition has been seemingly defined as serum sodium concentration < 130 mmol/L in Fig. 1. Both the prevalence of hyponatraemia in the general population and the threshold for therapeutic correction of low serum sodium are obviously different when adopting 130 or 136 mmol/L as the lower limits of the reference range. Even more importantly, the lack of appropriate treatment in patients with serum sodium comprised between 131 and 134 mmol/L may carry the risk of worsening hyponatraemia, thus generating a meaningful risk of inducing iatrogenic cerebral oedema, up to death. We hence suggest that a more precise definition of hyponatraemia should complement the otherwise valuable article of Grant et al. Due to the uncertainty still surrounding the upper limit of the reference range of serum sodium concentration (2), universal consensus should be reached, to definitely understand whether a sodium value is normal or not.