 Should people who eat plant-based diets be tested for vitamin B12 deficiency? In my opinion, medical tests should only be ordered if the results, one way or the other, are going to change what you do. If your tests came back showing your B12 levels were low, what would you do? You'd ensure a regular, reliable source of vitamin B12. If your tests came back showing your levels were okay, what would you do? You'd ensure a regular, reliable source of B12 because you wouldn't want them to become not okay. So if it's not going to matter either way, I wouldn't worry about it with two exceptions. I recommend anyone with unexplained neurological psychiatric or developmental symptoms be tested, especially in infants, toddlers, vegans, and anyone over 50. And out of an abundance of caution, I've always tested all my pregnant and breastfeeding vegan patients just because the consequences of deficiency are so potentially devastating. Better than getting a serum B12 level drawn, though, which most doctors do, a methylbalonic acid level is a superior test for B12 deficiency, which can be blood or urine, you can just pee in a cup for it. Here's why measuring MMA levels is better. About 50 people started eating vegan. Within a few years, half became B12 deficient. But look at their B12 levels. All these vegans had functional B12 deficiency, despite normal levels of B12 in their blood, showing that MMA is a more effective test. Now there has been a case report published of someone with apparent B12 deficiency who had normal B12 and MMA levels, so we're always looking for a better test. And it looks like hollow TC is it, measuring hollow transcobalamin levels, known to be more sensitive and specific, meaning fewer false negatives and false positives. If your level is under 20, you start treatment. If it's over 30, you can be pretty sure you're okay. And if it's in the middle, you follow up with a second-line test. If you aren't deficient, you can be treated with a ruby-red med-scientist-looking injection of B12, the color is from the cobalt in the molecule. But one of medicine's best kept secrets is the efficacious use of high-dose oral B12—safer, cheaper, 2,000-microgram supplements every day for two weeks should do it before having patients starting or resuming their regular, reliable regimen of B12.