A ferritin test measures the amount of a blood protein called ferritin, which stores iron. 

Your body  uses some of the iron it gets from food right away to make oxygen in your blood. But it also keeps some iron on hand for times when you don't get enough from the food you eat.

The iron is stored in a protein called ferritin. To see how much is in your body, your doctor might order a ferritin blood test. The test also can offer clues on conditions not linked to your iron levels. You might get other blood tests as well, including one that looks directly for iron in your bloodstream.

Iron is a mineral you get from food, while ferritin is a blood protein that stores iron. But testing for ferritin is one way your doctor tells whether you're storing a healthy amount of iron in your body.

You usually get a ferritin blood test because your doctor suspects you have too little or too much iron in your body. That might be because of your symptoms or because other blood tests suggest a problem. You also might get a ferritin test if you're at a risk of low iron levels because you are underweight or pregnant or have heavy periods, a poor diet, or problems with food absorption.

The test can help find  conditions including:

Iron deficiency anemia. You need iron to make healthy red blood cells, which carry oxygen through your body. When you don't have enough iron, you don't make enough of those cells. That's anemia. The most common cause of iron deficiency anemia in adults is chronic blood loss.

Hemochromatosis or iron overload. That's when you have too much iron in your body because you absorb too much from the foods you eat. It's a genetic condition.

Liver disease. Much of the ferritin in your body is stored in your liver. High ferritin can point to a liver problem.

Adult Still's disease. This rare form of arthritis causes joint pain, fever, and rash. People with this disease have very high ferritin levels.