What makes an a bone marrow biopsy abnormal?

Change in components. A bone marrow biopsy in a normal person will find the pattern of cells most often seen in a healthy person of that age.This would include red cells and their precursers (builders), white cells & precursers, platelet precursers & others. In illness, the pattern changes, and some or all may be abnormal. Tumor cells may take over every space& or few if any normal cells may be seen.