Relationship between lithium dose and serum concentration

Lithium is one of the agents used in patients with affective disorders, is also known as a "mood stabilizer". As lithium therapy is characterized by a narrow therapeutic range associated with significant side effects, it requires continuous therapeutic drug monitoring. Fasting serum lithium concentration was determined in 75 patients treated at the Department of Psychiatry, Sestre milosrdnice in University Hospital, administered 300, 600 or 900 mg lithium per day. The patients were divided into three groups according to drug dose: 300 mg (A), 600 mg (B), and 900 mg (C). The mean (range) level of the lithium was 0.40 (0.24-0.59 mmol/L) i.e. below therapeutic range (0.5-1.2 mmol/L) in group A (n=9), 0.70 (0.33-1.17 mmol/L) in group B (n=18), and and 0.66 (0.2-1.1 mmol/L) in group C (n=48).In group A, only one of nine patients had a serum lithium level within therapeutic range. In the remaining eight patients the dose was increased, as they had failed to respond to the initial one. In group B, three of 18, and in group C nine of 48 patients failed to improve on initial doses. Their serul lithium level was below therapeutic range and their hospitalization continued. Serum lithium levels >0.9 mmol/L were recorded in six and five patients from groups B and C, respectively. These patients experienced major undesired effects including ataxia, vertigo, nystagmus, muscle weakness, lethargy and thirst. In these patients, frequent control visits were advised (psychiatrist inteview, serum lithium level).. Study results pointed to the importance of lithium concentration monitoring in patients with affective disorders.