The unsaturated binding capacity (UIBC) is an indication of how many "seats" are available on transferrin, the iron-binding molecule that circulates in the blood. Under normal circumstances, approximately one third of the sites available on transferrin are saturated with iron, leaving two thirds unoccupied or "unsaturated."
However, a decrease in iron availability will increase the unsaturated binding capacity because there are more empty seats available. On the other hand, when there is too much iron around, the UIBC will decrease because there are fewer empty seats available.
Measuring the UIBC can help in the diagnosis of iron deficiency or iron overload though it is not a standalone biomarker. Levels should be assessed along with serum iron, ferritin, and transferrin biomarkers.
Vitamin B12 status should also be evaluated as a B12 (cobalamin) deficiency can mask an iron deficiency. In the event of cobalamin deficiency, the values of iron status biomarkers can be falsely elevated or decreased accordingly.
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