Is there a test to measure intracellular level b12? some variants transcobalamin ii are ineffective in transporting it across the cell membrane even when serum levels are normal. a functional deficiency. test would help determine treatment?

If it's at all possible, I would think it's only done by research studies & labs. B12, orally,sublingually or parenterally is a very innocuous substance. Excessive amounts are simply excreted in the urine. Therefore, if there's any doubt, I vote for administering it in strong doses & frequency although I wasn't familiar with transport issues at the cellular level.

There is not test for intracellular B12. Functional deficiency of B12 can be assessed by urine test for methylmalonic acid. Vitamin B12 deficiency leads to a buildup of methylmalonic acid (MMA) in urine. Urine MMA is a specific indicator of functional B12 deficiency and may indicate deficiency sooner than a B12 blood test. However, no test will address your irrational concerns.