Brief Answer:
Ferritin test

Detailed Answer:
Hi
XXXXXXX
Iron deficiency is the most common cause of anemia.
Your brothers iron is low inspite of using iron supplements may be due to his not getting enough iron in food or if he is having bleeding inside his body. This bleeding may be caused by problems such as ulcers, hemorrhoids, or cancer.This bleeding can also happen with regular aspirin use.
Other causes include if he cannot absorb iron well in his body. This problem may occur if he has celiac disease or if he had part of his stomach or small intestine removed.
I would suggest the following investigations for your brother
A complete blood count (CBC), to look at the shape, color, number, and size of the blood cells.
Iron tests, which measure the amount of iron in his blood, to help determine the type and severity of anemia.
Reticulocyte count, to help determine the cause of anemia. Reticulocytes are immature red blood cells produced by bone marrow and released into the bloodstream. Levels of reticulocytes are lower in iron deficiency anemia.
A ferritin level test, which reflects how much iron may be stored in the body. Abnormally low ferritin levels may point to iron deficiency anemia. This is one of the first tests to be abnormal when you have iron deficiency.

If your doctor suspects that bleeding in his stomach or intestines is causing your anemia, you will have tests to determine the cause of the bleeding. These may include:
A fecal occult blood test (FOBT), which looks for blood in stool samples.
A colonoscopy. This test inspects the entire large intestine (colon) using a long, flexible, lighted viewing scope to look for polyps or other sources of bleeding.
An upper gastrointestinal (GI) endoscopy. This test, which uses a thin, flexible, lighted viewing instrument, can help identify stomach ulcers or other causes of irritation or bleeding.
Video capsule endoscopy. For this test, you swallow a capsule that contains a tiny camera. As the capsule travels through your system, the camera takes pictures of your small intestine that can show where bleeding is occurring.
X-ray tests such as an upper GI series or barium enema.

If blood tests don't find the problem, he may need a test called a bone marrow aspiration. Bone marrow aspiration removes a small amount of bone marrow fluid through a needle inserted into the bone. Because iron is stored in the bone marrow, this test can provide a good idea of how much iron is in the body. But bone marrow aspirations are not done very often.

Hope I have answered your query

Regards
DR De

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