Canadian Journal of Medicine

In 2019, Shulman et al. [1] summarized the known lithium therapeutic ranges for Old Age Bipolar Patients (OABP).  For patients over 60, the range is 0.4-0.8 mmol/L, and for those over 80, 0.4-0.7 mmol/L. For younger patients, the standard region is 0.6-1.2 mmol/L. The current research leaves three disconnected therapeutic ranges. A unifying theory is developed explaining the therapeutic range of lithium for all ages. It explains why the changes in the therapeutic range happen and exactly how the upper limits change on a year by year basis instead of by decades.  It also explains why and when the standard therapeutic range should be abandoned.   In developing the theory, only information about the changes with age in the eGFR is used; when applied to lithium, the theory explains why the therapeutic regions for over sixty and over eighty exist. The known ranges were discovered using empirical evidence. The theory gives a more precise start for those regions as well as how to move smoothly through them.  It applies for bipolar patients who do not have chronic kidney disease and shows that a new therapeutic region for those over 90 needs to be created. Table 1 is developed using the new theory to give the upper limit, the midpoint, and the twenty-fifth percentile up from the minimum level of the therapeutic range for each year from age fifty to one hundred.  This table enables doctors to keep the patient’s serum level where they want it as the patient ages.

The case follows the treatment of a woman from age 73 to age 83 who was bipolar and who also had Chronic Kidney Disease (CKD). During the study, the eGFR dropped from 43 to below 20. Initially the treatment was fairly standard but highlighted the difficulty of knowing when to reduce the lithium dose before undesired mood changes occurred. In phase 2 of the treatment, the usual monthly blood serum test is combined with a new concept of a monthly running average eGFR to decide when to reduce the lithium dose. A new way of administering lithium was created which was used when the patient’s daily dose of lithium reached 300mg a day.  In place of a fixed daily dose of lithium, a fixed average daily dose of lithium over a short cyclic pattern was used. The daily dose varies within the repeatable cycle but is smoothed out by the long half-life of lithium. The new method allows for the reduction of the lithium dose by small amounts, less than the minimum strength 150mg capsule used in North America or the 100mg tablet used in Europe.  The method has been successfully used for several years with the patient’s moods stable and lithium toxicity avoided. The patient is currently on an average daily dose of 187mg a day with an eGFR less than 20.