How does a doctor determine the correct dosage of Teretol Retard?

I am taking Tegretol Retard for epilepsy - 200mg twice a day. I keep reading that the minimum effective dosage is 800mg a day. Is this correct or is it dependent upon the individual? Also how does the doctor determine the dosage? Is it done through blood tests or is it simply a case of trying to see how much is effective?|||ITS DEPENDED ON EACH AND EVERY INDIVIDUAL|||200mg twice a day is the recommended starting dose. The minimum effective dose of 800mg per day is just a guideline where most people start to see improvement. The correct dose for you is the lowest dose that controls your seizures whether it's 400mg per day or the maximum of 1600mg per day. Blood levels are the next most important parameter. A Tegretol (carbamazepine) blood level of between 4-12 mcg/ml is effective for the majority of patients. The dose is determined by starting with 200mg twice a day and slowly increasing it, until a favorable response is seen, up to the maximum daily dose.