Serum lithium concentrations around the clock with different treatment regimens and the diurnal variation of the renal lithium clearance.

The serum lithium concentration was determined around the clock in patients treated with conventional tablets given once daily, in the evening, and in patients treated with slow-release tablets given twice daily, in the morning and in the evening. Curve shapes differed markedly in the two groups, with much wider variation of serum concentrations in the former than in the latter. The data were used to calculate for the two patient groups the ratio of the mean serum lithium concentration over the 24-h day to the serum lithium concentration in blood samples drawn 12 h after the last intake of lithium. Around-the-clock determinations of the patients' renal lithium clearance showed about 20% lower values during the night than during the day.