What is the biggest cause of low blood count?

It depends upon....: The blood counts to which you are referring. If you mean anemia, the overwhelmingly most common reason for a low rbc count in the us is fe deficiency, in part because of our typically fe poor diet, and exacerbated by a monthly blood loss in women. Fe deficiency, however, is due tof an underlying primary problem to be discovered and corrected. For low WBC count, viral suppression is most common.