The therapeutic level of a drug in the bloodstream is the range within which that drug is expected to be effective without causing any serious problems to the patient. Your doctor can request a test to measure the amount of a specific drug in the serum portion of your blood. Your doctor will check those levels against established ranges for that drug to determine if the value is therapeutic or not.
The most important bipolar disorder drugs to be monitored regularly are these three mood stabilizers:
Medline Plus Medical Encyclopedia. Therapeutic drug levels. US National Library of Medicine and National Institutes of Health. 24 Aug 2009.