Seeing questions about how to tell if Vitamin B12 is getting into the cells. There is a test from a company called SpectraCell that can check your vitamins at the CELLULAR LEVEL. Have done this twice and it's what finally helped my doctor figure out why the heck I was so exhausted all the time. Although the B12 in my blood tests showed normal, or even high, at the cellular level was a totally different picture. I was found to be FUNCTIONALLY DEFICIENT in seven vitamins, most all the B's. This is lower than low range. You have some blood drawn and send it off. It takes almost a month to get the test results because of all the work and testing they do. They are a great company to work with and I highly recommend this test to anyone who's level in the blood is normal but still feels so exhausted. It may shed some light on other conditions!