What is the optimal serum valproic acid (valproate) level?

Serum valproic acid levels in patients with bipolar disorder are typically kept anywhere between 50 mcg/mL and 125 mcg/mL and it is not always clear why a particular serum level was chosen.  More importantly, I have seen many, many patients who were treated with divalproex at serum levels in the 50s or 60s and were…