In practical terms, total iron binding capacity=transferrin. Why?
TIBC refers to the amount of iron in the serum + the amount of iron that could theoretically still be bound to transferrin. Transferrin is usually only 25-30% saturated at any given point.
Knowing the molecular weight of the transferrin (80 kDa) and that each molecule of transferrin can bind 2 atoms of iron, TIBC and transferrin concentration is interconvertible. If you want to get into the mechanics of it, here is a somewhat dotty conversation from the UK. I have to admit I still didn’t understand it after reading those series of equations.
If it is possible to order transferrin and TIBC separately, transferrin may be useful as a marker of nutritional status or liver status, since it is produced in the liver.
Side note: ferritin, which is completely different, is usually correlated with how much iron is stored in tissues. However, because it’s an acute phase reactant, it may be falsely elevated in inflammation or anemia of chronic disease.