There is a therapeutic level that your doc is trying to achieve. I found the information below from a site about Depakote.
The amount of valproic acid in the blood can be measured. Studies have shown that blood levels between 50 – 125 (micrograms/mL) give patients the best chance of response to valproic acid. In the beginning of treatment, your doctor may check your blood once or twice a week. Once your symptoms are controlled well, blood samples are drawn less frequently.