From my understanding, to accurately test for b12 serum you need to be off of b12 supplements and b vitamin drinks for at least 4 months (the average life of a red blood cell) . There are other tests to be ran too, but there is no gold standard test. Plus, there is research that the "normal" cut off of above 200 is too low, especially if neurological symptoms are present. Have you heard of the book Could It Be B12? I listened to it for free thru my local library and the free app Hoopla. You can also watch the free movie Sally Pacholok on YouTube. She also has a website. From my understanding, if you have neurological symptoms you need every other day b12 injections until symptoms improve. I am going thru similar issues of buzzing and tingling in my leg. I am also getting the run around in doctors. I have lots more to say on this. But I'll stop here while you have time to process and research what I've said. I've learned I have to advocate and fight for my life.