Anemia is a common cause of fatigue. 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.This test helps determine if anemia is caused by bleeding (GI tract or mensuration) or due to other Causes