Intensive care-acquired hypernatremia is usually managed with a positive fluid balance [1]. However, extensive evidence indicates that overly positive fluid balances are associated with intensive care unit (ICU) morbidity and mortality [[2], [3], [4]]. Hypernatremia is common in patients admitted to the ICU and is an independent risk factor for in-hospital morbidity and mortality [[5], [6], [7], [8], [9], [10], [11]]. Common causes of hypernatremia in critically ill patients include lack of access to water, osmotic diuresis, and hypotonic losses [12].

Hypernatremia has also been reported during the recovery from acute kidney injury (AKI), likely within the context of sodium overload during the initial fluid resuscitation [[13], [14], [15]]. This phenomenon may be explained by an impairment in the kidney's urine concentration capacity [15,16]. However, current evidence on this subject is restricted to case reports, case series, and retrospective studies [12]. Additionally, patients recovering from AKI have a reduced capacity to concentrate urine and may not benefit from positive fluid balances [15,17].

In this prospective cohort study, we sought to describe: 1) the trends in plasma sodium in critical care adult patients recovering from AKI, and 2) the impact of fluid balances on plasma sodium. We hypothesized that there is an association between fluid balances and sodium trends during kidney recovery. As a secondary objective, we aimed to describe the incidence and risk factors for plasma sodium increase and hypernatremia during the recovery from acute kidney injury.