The red blood cell/plasma lithium ratio: marker of biological heterogeneity within bipolar affective illness?

The red blood cell/plasma concentration ratio (LR) was studied in 45 female patients with bipolar affective illness who responded to prophylactic lithium treatment. Patients were subdivided according to bipolar I vs. bipolar II diagnosis and presence vs. absence of a family history of affective illness. Mean LRs were significantly higher in bipolar II patients and in those with a positive family history of affective illness, but there was no relationship between diagnosis and family history. Results indicate that LR is genetically controlled and may be a biological marker distinguishing bipolar I and bipolar II patients, and also patients with positive or negative family history of affective illness.