A total iron-binding capacity (TIBC) test measures the blood's ability to attach itself to iron and transport it around the body. A transferrin test is similar.
If you have iron deficiency ↗ (a lack of iron in your blood), your iron level will be low but your TIBC will be high.
If you have too much iron (for example, if you have a condition like haemochromatosis ↗), your iron level will be high but your TIBC will be low or normal.
Transferrin is a protein produced by the liver. It regulates the absorption of iron into the blood. TIBC relates to the amount of transferrin in your blood that's available to attach to iron.
Although TIBC and transferrin are 2 different tests, they basically measure the same thing, so you'll usually have either one or the other.
As transferrin is produced by the liver, your TIBC level will also be low if you have liver disease ↗.
Read more about the TIBC test ↗ at Lab Tests Online UK.