The normal range for B12 concentrations in the blood are 200 to 900 picograms per milliliter of blood, states Medline Plus. Any level above 900 or below 200 may point to an underlying condition.
If a patient has too little B12 in her blood, then she may have B12 vitamin deficiency, states Medline Plus. This type of vitamin deficiency may occur in vegetarians and vegans, in patients with malabsorption diseases or in pregnant women. A B12 concentration of 900 picograms per milliliter is very uncommon, but it may occur in patients with certain liver diseases or with certain myeloproliferative diseases such as polycythemia.
A range of 81 to 1107 picomoles per liter is also normal for vitamin B-12 levels. Vitamin B-12 levels are tested with a blood sample taken from a vein in the arm. This test is usually painless and may result in some mild discomfort or slight bruising at the sample site.