Functional medicine uses specific labs that identify how your body is functioning.  Instead of only testing the quantity of a certain vitamin in the blood, we will evaluate the biproducts of the vitamin in your urine in order to find out whether it is being used properly.  This type of information paints a much clearer picture of the biochemistry of your body, specifically. Rather than assuming that you are functioning the same as every other human with the same level of said vitamin. Because we already know, that idea is a falsehood in medicine.