There are 4 main tests for vitamin B12

1. Serum B12 – The normal range is 200 to 900 pg/ml. (Please note that your B12 may have been measured by another system – pmol/l so you should see the normal range described in your report.) If your B12 is less than 150 pg/ml it is very low. A good level is around 400 pg/ml. If a cyanocobalamin source (like spirulina or chlorella) is consumed these levels may be high but the B12 is still not usable by the body. In this case, it’s better to rely on one of the other two tests or do the homocysteine levels along with the vitamin B12 levels.
2. Homocysteine level – High homocysteine levels are an indicator that the usable vitamin B12 is low, and should be supplemented. If serum B12 levels are normal but homocysteine is high (common in people taking cyanocobalamin supplements), a methylcobalamin supplement is necessary.
3. MMA – Methylmalonic Acid, serum: MMA, along with homocysteine, helps diagnose an early or mild B12 deficiency. If MMA and homocysteine levels are increased, then vitamin B12 deficiency may be present, indicating less available B12 at the tissue level. If only homocysteine is elevated, then folic acid may be low or not metabolised properly. If MMA and homocysteine levels are normal, it is unlikely that there is a B12 deficiency.
4. Holotranscobalamin– this test is not done everywhere and is expensive but it’s the best test because it measures usable B12.

Under normal circumstances it is alright to rely on serum B12 if cyanocobalamin supplements are not being taken. Otherwise, homocysteine levels are more reliable. Alternatively, both serum vitamin B12 and homocysteine levels can be tested.