And one worth testing for beyond the standard B12 serum testing--as Rose and Mrs. D have been pointing out for years, the serum measurement is just the grossest of tests, only revealing the amount of B12 currently in the bloodstream--not in the tissues where it is most needed. The homocysteine and MMA tests are better indicators of B12 deficiency--not perfect indicators, but better.

Two things should be mentioned, however--injections are not the only way to go, as there is substantial evidence that sublingual mass B12 dosing can help rebuild stores just as well, and is far cheaper. (The reason for the mass dosing, in amounts 1000mcg or higher, is to allow for the 1-2% passive absorption capable even in those with no intrinsic factor to break the vitamin out of foodstuffs.) Secondly, many of the injections out there are composed of the cyanocobalamin form of B12 (as are many commercial supplements), and while most can rebuild stores on that, some who have inherent errors pertaining to methylation biochemistry would do better on one of the other forms, such as methylcobalamin, which doesn't have to jump through as many metabolic hoops to be utilized.