Can there be different opinion on beta 2 microglobulin and bone marrow biopsy regarding detection on multiple myeloma?

Probably not: The presence of more than 10% plasma cells in the bone marrow is supportive of the diagnosis. Once the diagnosis is confirmed the level of b2m is very important in terms of prognosis.

Beta 2 Microglobulin: Is a way of following myeloma response to therapy. The problem is that it can be elevated in other conditions- including benign ones. I've seen it very high in patients with kidney dysfunction. So the leel could be different than the bone marrow, which is the only real way to prove multiple myeloma.