Diagnosing anemia involves determining if the number of RBCs or level of hemoglobin in your blood is low. Often, once you have been determined to have anemia, further steps may be necessary to determine what type of anemia you have because proper treatment for anemia depends on the correct diagnosis of its cause.

A doctor will most likely begin by taking your personal and family history and performing a physical examination. Next, he or she will take a small blood sample for testing. A panel of tests called the complete blood count (CBC) measures the number of RBCs in the blood along with their size, shape, color, and hemoglobin content. If your RBC count or your hemoglobin level is too low, it indicates you have anemia. The appearance of the cells can also help illuminate the cause. For example, if the cells are small and pale, it may indicate iron-deficiency anemia, while large cells can suggest some type of vitamin-deficiency anemia.

Another test that can be run on a blood sample to help figure out the cause of anemia is hemoglobin electrophoresis, which can detect abnormal hemoglobin in the RBCs such as those found in sickle cell anemia or the thalassemias. Another test called a reticulocyte count, which measures the numbers of immature RBCs called reticulocytes in the blood, can determine whether your body is producing RBCs at the proper rate. People with pernicious anemia have low reticulocyte levels. Tests can also determine if you are deficient in iron, B12, or folate.

The doctor may also want to check if you are bleeding internally. A stool sample can be used to determine if gastrointestinal bleeding could be causing your symptoms. If blood is detected in the stool, an endoscopy may be performed which uses a small fiber optic camera to examine the digestive tract. 

In certain cases, a bone marrow biopsy may be required to test for problems with the bone marrow. This usually involves inserting a needle into the pelvic bone or another site to remove a small sample of bone marrow tissue.