If vitamin B12 deficiency anemia is suspected, your doctor will do a physical exam and ask questions about your medical history and symptoms. You will also have blood tests, such as:

• Complete blood count (CBC). This test gives important information about the kinds and numbers of cells in the blood. Having a low red blood cell count is a sign of anemia. Vitamin B12 deficiency anemia causes the red blood cells to be larger than normal, so it is called a macrocytic (meaning big cells) or megaloblastic (big, immature-looking cells) anemia. Problems other than vitamin B12 deficiency anemia also can cause macrocytic anemia.
• Vitamin B12 test to measure the level of this vitamin in the blood.
• Folic acid test. Folic acid is another type of B vitamin. Some people who have vitamin B12 deficiency anemia also have folic acid deficiency anemia, and both of these deficiencies cause similar symptoms.
• Tests to detect the presence of certain antibodies that may help diagnose pernicious anemia.
• Homocysteine test and methylmalonic acid (MMA) test. The amount of these substances in the blood rises as the level of vitamin B12 decreases. Your doctor may use these tests to check to see why vitamin B12 levels may be borderline low.

Having pernicious anemia may increase a person's risk of developing stomach cancer. If you have pernicious anemia, you may need to ask your doctor whether you should be screened for stomach cancer on a regular basis.