Lithium dosage in the elderly. A study with matched age groups.
Lithium dosage was examined in 3 age groups (less than or equal to 45 years, 46-64 years, greater than or equal to 65 years) of 78 inpatients, matched on relevant variables. Although mean steady-state lithium plasma levels showed no essential differences, mean daily lithium doses were significantly lower in the elderly. The ratio of weight-related lithium dose to plasma level showed a 36% decline with age (0.58, 0.46 and 0.37 1/kg, P less than 0.0001, analysis of variance). Comparing different decades this ratio, indicating the dose required for a certain plasma level, displayed a clear-cut decrease at 50 years. This may be caused by age-related changes in apparent volume of distribution and/or in elimination half-life of lithium, the latter being the consequence of age-related decrease of glomerular filtration rate.