Background A wide range of interesting mathematical models has been derived to predict the effect of intravenous fluid therapy on the serum sodium concentration (most notably the Adrogué–Madias equation), but unfortunately, these models cannot be applied to patients with disorders characterized by aberrant antidiuretic hormone (ADH) release, such as the syndrome of inappropriate ADH secretion (SIADH). The use of intravenous fluids in these patients should prompt caution, as the inability of the kidneys to properly dilute the urine can easily result in deterioration of hyponatremia. Methods In this report, a transparent and clinically applicable equation is derived that can be used to calculate the estimated effect of different types and volumes of crystalloid infusate on the serum sodium concentration in SIADH patients. As a “proof of concept”, we discuss five SIADH patient cases from our clinic. Alternatively, our mathematical model can be used to determine the infusate volume that is required to produce a certain desired change in the serum sodium concentration in SIADH patients. Conclusion The presented model facilitates rational intravenous fluid therapy in SIADH patients, and provides a valuable addition to existing prediction models.

Objective: To investigate the occurrence of disorders of water and sodium balance in COVID-19 in our clinic. Methods: In this retrospective chart review, patients were included if a polymerase chain test result for SARS-CoV-2 was obtained and if at least one plasma sodium concentration measurement was obtained during the period from March to June 2020. The occurrences of hyponatremia and hypernatremia were compared between 193 SARS-CoV-2-positive and 138 SARS-CoV-2-negative patients. A χ² test was used to determine statistical significance, and the corresponding p-values were calculated. Results: Hypernatremia was significantly more frequently observed in COVID-19 patients, in 38% (74 of 193), versus only 8% in SARS-CoV-2-negative patients (11 of 138) ( p < 0.01). Hyponatremia was observed in 34% of the included COVID-19 patients (65 of 193) versus 24% of SARS-CoV-2-negative patients (33 of 138). In 12% of all COVID-19 patients (23 of 193), both hyponatremia and hypernatremia were observed at some point during their admission. Among the non-COVID-19 patients, only 4% showed these plasma sodium concentration fluctuations (5 of 138). The mortality rate among the hospitalized COVID-19 patients was 23% (45 of 193). Correcting for double-counting, more than 71% (32 of 45) of the deceased COVID-19 patients developed dysnatremia (hyponatremia, hypernatremia or both) versus 57% (84 out of 148) of the surviving COVID-19 patients. Conclusion: Disorders of water and sodium balance—and especially hypernatremia—seem to be a common occurrence in COVID-19 patients. This has important implications for the treatment of COVID-19 patients.

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