Doctors use an important blood test called a complete blood count, or CBC for short, to measure the number of each blood cell in your blood sample. If the CBC shows a low number of red blood cells, white blood cells, or platelets, your doctor may look at the cells under a microscope. This is called a blood smear, and it can show if any blood cells are abnormal.
Low blood counts can have many causes, including vitamin deficiencies, bleeding, and rare bone marrow failure diseases like aplastic anemia, MDS and PNH. If your blood tests are abnormal, your doctor may do other blood tests or take a sample of your bone marrow to find out why.