INTRODUCTION: Hyponatraemia is associated with increased mortality in patients undergoing maintenance haemodialysis. In anuric patients, hyponatraemia development depends on the water-sodium ratio in retained fluid within the interdialysis interval (IDI). OBJECTIVE: This study aimed to calculate the retained sodium-retained water ratio in patients on maintenance haemodialysis and make a differential diagnosis of hyponatraemia according to these data. METHODS: The amount of retained water was determined as body weight gain (ΔBW) within the IDI. Sodium retention was calculated using our formula: eRNa+ = ΔBW × (SNa+)t2 - total body water (TBW)t1 × ([SNa+]t1 - [SNa+]t2), where TBW represents the calculated volume of the total body water and (SNa+)t1 and (SNa+)t2 represent the sodium concentration at the beginning and at the end of the IDI, respectively. We performed 89 measurements in 32 anuric patients on maintenance haemodialysis. RESULTS: Hyponatraemia was detected in 13 measurements at the end of the IDI. The ΔBW had no statistically significant difference between normonatraemic and hyponatraemic patients. Hyponatraemic patients had significantly lower levels of retained sodium. The retained water--retained sodium ratio facilitated in differentiating dilution hyponatraemia, nutritional hyponatraemia, depletion hyponatraemia, and dilution hyponatraemia associated with sodium wasting or malnutrition. CONCLUSION: The composition of retained fluid during the IDI may be hypotonic, hypertonic, or isotonic in relation to the extracellular fluid. Most of the hyponatraemic patients had hypotonic fluid retained during the IDI because of dilution as well as gastrointestinal sodium loss and/or malnutrition.

2nd faculty of Medicine Charles University Prague Prague Czechia

Department of Internal Medicine 2nd faculty of Medicine Charles University Prague and Motol University Hospital Prague Czechia

Haemodialysis Centre Fresenius NephroCare Motol Prague Czechia

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