Ferritin, serum iron and TIBC: what iron test results mean
A simple guide to the iron tests doctors order, what ferritin and transferrin saturation show, and why results can be misleading during illness.
Why iron is tested
Iron is needed to make haemoglobin. When iron runs low, the body first uses up its stores and later the haemoglobin falls. Iron tests help show where you are in that process.
The common tests
- Ferritin: reflects the body's iron stores. A low result is a reliable sign of low stores.
- Serum iron: the iron circulating in the blood at the time of the test. It varies through the day and after meals or supplements.
- TIBC (total iron-binding capacity) or transferrin: shows how much "carrying capacity" the blood has for iron. It tends to rise when iron is low.
- Transferrin saturation: the percentage of the carrying capacity that is in use. It is calculated from serum iron and TIBC.
Why results can be tricky
Ferritin is also an "acute phase" protein. It can rise with infection, inflammation, liver problems and other conditions, which can make iron stores look better than they are. That is why a doctor may combine ferritin with transferrin saturation, the CBC and your symptoms.
What to ask your doctor
- Are my iron stores low, normal or hard to judge?
- Could inflammation or another illness be affecting these results?
- What is the likely cause of low iron, and should anything else be tested?
- When should the tests be repeated?
Taking iron before a test
If you recently took iron tablets, tell the person taking your blood. They can change serum iron results for a short time.
If you would like help understanding your own results, see reports review or book a new consultation.
Frequently asked questions
Is ferritin the best test for iron deficiency?
Ferritin is the most commonly used marker of iron stores, and a low value is a strong sign of iron deficiency. But ferritin can rise with infection or inflammation, so doctors look at it alongside other tests.
Should I fast before an iron test?
Serum iron changes during the day and after meals or iron tablets. Follow the instructions given by the laboratory or your doctor, and tell them if you recently took iron supplements.