Vitamin B12 levels are tested in different ways with different accuracies.
Serum B12 by radioimmune assay (RIA) is less accurate than microbial assay since it picks up all forms of cobalamin including those that are inactive. Serum B12 by microbial assay appears to be the most widely used and is considered accurate. However, pregnancy, large doses of vitamin C, and folate deficiency may result in a falsely reduced B12 microbial assay. A 24-hour urine MMA (Methylmalonic acid) test is claimed to be very sensitive but is also more expensive. Without B12, MMA increases in the urine.
Another testing pattern used by doctors to determine vitamin B12 status is to test for intrinsic factor (IF) antibodies. Their presence prevents the normal binding of B12 to IF and thus prevents B12 absorption. Of pernicious anemia sufferers, 70% have these antibodies. If the test for IF antibodies is negative, a Schilling test is usually performed to help distinguish the nature of the problem. This test involves the use of radioactively labelled B12 and may be objectionable to some people.
A lack of gastric hydrochloric acid tends to confirm a suspected lack of intrinsic factor (IF), as both can be due to a shrinking of gastric cells.
During the Schilling Test, the patient's B12 binding proteins are saturated by giving a large dose of unlabeled B12 intravenously. Absorption (via the ileum) of cobalamin is measured following ingestion of radioactively labelled B12.
In a normal system, more than 10% of the radioactive cobalamin will be excreted in the urine over 24hours. If less than 10% is excreted, the patient is given cobalamin + IF. If the patient is suffering from Pernicious Anemia, greater than 10% of the labelled cobalamin will appear in the urine within 24hours, otherwise the patient can be said to have an absorption problem.
Low results may occur in patient's with kidney problems.
A vitamin B12 deficiency is the most common cause of megaloblastic anemia. When testing facilities are not available or cannot be afforded, intramuscular or sublingual B12, with or without folic acid, can be used to see if symptoms improve.
If vitamin B12 deficiency is suspected, tests to measure the blood level of B12 are routine. Usually, the presence of the intrinsic factor (IF) is determined by testing for the presence of antibodies to the IF in the blood. Gastric analysis may be required to confirm the presence or absence of IF.
Taking a lot of folic acid may cover up the B12 anemia and other symptoms until it is too late for effective treatment with vitamin B12. Therefore, vitamin tablets of folic acid with over 400mcg have been taken off the market and are available by prescription only. If megaloblastic anemia occurs, both folic acid and vitamin B12 levels should be checked to assure proper treatment and follow-up.
For raw-food vegans who have not been careful about consuming enough B12 (in the form of supplements or fortified foods), it would be wise to have B12 levels assessed and to commence dietary supplementation immediately. Some 80% of those who have been vegan for over 2 years have a functional B12 deficiency – the figure for raw food vegans is almost certainly higher, unless they make a point of not excessively washing their fruits and vegetables.
For longer-term vegans who have not been too careful about consuming enough B12 (in the form of supplements or fortified foods), it would be wise to have B12 levels assessed and to commence dietary supplementation immediately. Some 80% of people who have been vegan for over 2 years have a functional B12 deficiency, meaning their homocysteine is too high.
For longer-term vegetarians who have not been careful about consuming enough B12 (in the form of supplements or fortified foods), it may be wise to have B12 levels assessed and to consider dietary supplementation. About 25% of all ovo-lacto vegetarians have a functional B12 deficiency, meaning their homocysteine is too high.