Lithium
Lithium is a light metal that is used specifically for the treatment and prophylaxis of bipolar depression (manic- depressive disorder). The exact mechanism by which lithium stabilizes mood is unknown. The psycotrophic effect of lithium is not seen in healthy people. Lithium is usually taken orally, usually with meals or as a slow-release form. Absorption and distribution are slow, taking up to 10 to 12 hours for complete distribution of a dose. The blood half-life is 24 to 40 hours. Lithium is not metabolized and is excreted entirely by the kidneys. The full therapeutic effect may not be seen for 7 to 10 days. After the initial steady state has been achieved, serum lithium levels should be measured daily to establish a mean concentration between 0.5 and 1.5 meq/L. When the desired therapeutic level has been achieved, lithium should be measured once a week for 4 weeks and then every other month thereafter. Renal and thyroid function should be monitored periodically because lithium can cause hypothyroidism and renal dysfunction. Lithium therapy may increase total and ionized serum calcium and magnesium levels.
Bipolar and unipolar depressions are often successfully treated with lithium concentrations between 0.8 and 1.0 meq/L, while manic patients often require higher concentrations of 0.9 to 1.4 meq/L. The lithium therapeutic range is narrow. The relationship between trough serum concentration and toxic symptoms is summarized in the table below.
|
Concentration |
Symptoms |
|
1.5 - 2.5 |
Drowsiness, nystagmus |
|
2.5 - 3.5 |
Psychoses, facilitations |
|
>3.5 |
Convulsions, stupor, coma, death |
Mild toxicity is usually seen in the 1.5 to 2.5 meq/L range and severe toxicity occurs over 2.5 meq/L. Excessive sweating, fever, weight reduction and low salt diets may increase lithium levels. Methyldopa, L dopa, and diuretics may also increase lithium concentration.
Blood should be drawn 12 hours after the last dose. Therapeutic (trough) range is 0.5 -1.5 mEq/L (Vitros analyzer). The critical value for lithium is 2.0 mEq/L.
Specimen requirement is one SST tube of blood. Trough levels should be drawn before the next scheduled morning dose, 8 to 12 hours after the last dose.
