The relationship of plasma to erythrocyte lithium levels in patients taking lithium carbonate.

Plasma and erythrocyte lithium levels have been determined repeatedly in twelve patients taking lithium carbonate for affective disorders. In any individual the plot of the plasma lithium level against erythrocyte/plasma ratio is linear, but the ratio can either increase or decrease with increasing plasma lithium concentration. Erythrocyte/plasma ratio is an unsound basis for comparing individual responses to lithium.