Endocrine Abstracts

Aims: The lowest licensed dose of tolvaptan for treatment of hyponatraemia is 15 mg. There is little data on lower doses. Our study aimed to evaluate the safety of an initial dose of 7.5 mg tolvaptan.

Methods: We retrospectively reviewed data from a London teaching hospital over a 6-year period. All adults administered a first dose of 7.5 mg tolvaptan were included. Three different timeframes were reviewed: 4-12, 12-18 and 18-30 hours. We analysed response to a second dose of 7.5 mg or 15 mg tolvaptan.

Results: 181 patients met the inclusion criteria. Regardless of pre-dose sodium levels, 7.5 mg tolvaptan resulted in a significant increase in sodium at all time intervals, with a mean increment at 4-12 hours of 4.54 mmol/l (P < 0.0001), rising to 6.1 mmol/l by 18-30 hours (P < 0.0001). 137 patients had blood taken between 4-12 hours, 8.7% had a rise of 10 mmol/l or more. 93 patients had a blood test between 18-30 hours, 19.4% had an unsafe rise. 34 patients had an over correction, 33 had a basal level of ≤127 mmol/l . There were no instances of osmotic demyelination. 95 of the patients were given a second dose of tolvaptan during their admission, 55 were given 7.5 mg and 45 had 15 mg. There was not a significant difference between the two doses in their increase to sodium. 83.6% of the patients administered 7.5 mg showed a smaller increase in sodium than after their first dose, with 1 case of overcorrection. 73.3% of the patients administered 15 mg showed a smaller increase in sodium than after their first 7.5 mg dose, with 2 patients overcorrecting.

Conclusion: This is the largest study reviewing the use of 7.5 mg tolvaptan for treatment of hyponatraemia. We recommend an initial dose of 7.5 mg with monitoring of sodium levels at 8 and 24 hours post administration. If a second dose is required, 7.5 mg is safer than 15 mg.