Objectives: Little is known about how lithium should be dosed to achieve therapeutic but safe serum concentrations in older adults. In this paper, we investigate how the lithium dose-concentration ratio changes across the lifespan.
Methods: This was a cross-sectional analysis of 63 current lithium users aged 20-95 years using data from McGLIDICS (the McGill Geriatric Lithium-Induced Diabetes Insipidus Clinical Study). Participants underwent blood and urine tests, including serum lithium concentrations. Multivariate analyses were conducted to evaluate potential correlates of the lithium dose-concentration ratio.
Results: We found that between the ages of 40-95 years, the total daily dose of lithium required to achieve a given serum concentration decreases threefold (500 vs. 1,500 mg for 1.0 mmol/L). Greater age, once-daily dosing, and lower renal function (estimated glomerular filtration rate) were independently associated with a lower lithium dose-concentration ratio.
Conclusions: The lithium dose required to achieve a given serum lithium concentration decreases threefold from middle to old age, with this trend continuing into the ninth and tenth decades of life. In order to avoid lithium toxicity in aging patients, continued serum concentration monitoring and judicious dose reduction may be required, particularly in those patients with reduced renal function.