Of particular note:
Red Blood Cell (RBC)/Serum Lithium RatioEmphasises mine.
"The distribution of lithium across cellular membranes is controlled by several membrane transport and countertransport mechanisms (7). Among these, the sodium- lithium countertransport system appears to play a particularly pivotal role. The status of this membrane transport system is clearly under genetic control and is strongly associated with the risk for hypertension (81). This system also is significantly less active in African-Americans and African Blacks than in Caucasians, which might contribute to the higher prevalence of hypertension among Blacks (7). More recent studies have shown that, in addition to its hemodynamic implications, ethnic variability in the activity of this system also leads to significant differences in the RBC/serum lithium ratio (59, 60, 77). This ratio is likely correlated with the intracellular concentration of lithium, which might have important meaning not only in terms of the genetic control of cell membrane permeability to lithium, but also in terms of the clinical and side effects of lithium. Thus, the difference between Blacks and other ethnic groups in the RBC/serum lithium ratio might have important clinical significance. Such a possibility has been recently demonstrated by a study conducted by our group; this study revealed that significant differences in the lithium ratio exist between African-American and Caucasian bipolar patients, and it further demonstrated a higher rate of central nervous system (CNS)-related side effects in African-American patients, suggesting that the higher lithium ratio in this group might indeed lead to higher central toxicity (77). "