Biomarkers of Iron Status: UIBC

Optimal Takeaways

Unsaturated iron binding capacity (UIBC) reflects how much room is available on transferrin to transport iron in the blood. Usually, just one-third of transferrin is saturated with iron, leaving the remaining two-thirds “unsaturated” and available to pick up more iron. Measuring UIBC assists in the diagnosis of iron deficiency or iron overload. The UIBC increases in iron deficiency as less iron occupies the binding sites on transferrin, and the UIBC will decrease with iron overload as iron takes up more sites.

Standard Range: 110 - 350 ug/dL (19.7 - 62.68 umol/L)

The ODX Range: 130 - 300 ug/dL (23.28 - 53.73 umol/L)

Low UIBC is associated with highly saturated transferrin, excess iron, hemochromatosis, and hemosiderosis (Pagana 2021).

High UIBC is associated with iron deficiency anemia, oral contraceptives, pregnancy, and polycythemia vera (Pagana 2021).

Overview

Unsaturated or unbound iron binding capacity (UIBC) represents circulating transferrin's unoccupied iron binding capacity. Combining UIBC with serum iron (i.e., iron bound to transferrin) provides the value of total iron binding capacity. Measuring UIBC may be an inexpensive alternative to measuring transferrin saturation (Pagana 2021).

In an outpatient study of 48,903 subjects, measuring UIBC to assess empty iron stores was superior to serum iron, transferrin, and transferrin saturation, including in those individuals with elevated hs-CRP or low hemoglobin. A UIBC of 290 ug/dL (52 umol/L) or above should be investigated further to determine iron stores (Asberg 2013) best.

A vitamin B12/cobalamin deficiency can mask iron deficiency and interfere with the accurate assessment of UIBC. In a study of 75 patients with cobalamin deficiency, serum iron, ferritin, and transferrin saturation levels increased while UIBC decreased, suggesting iron sufficiency. After cobalamin therapy, serum iron, ferritin, and transferrin saturation decreased significantly while the UIBC increased significantly, suggesting iron insufficiency (Solmaz 2015).

References

Åsberg, Arne et al. “The diagnostic accuracy of unbound iron binding capacity (UIBC) as a test for empty iron stores.” Scandinavian journal of clinical and laboratory investigation vol. 73,3 (2013): 208-13. doi:10.3109/00365513.2013.765029

Pagana, Kathleen Deska, et al. Mosby's Diagnostic and Laboratory Test Reference. 15th ed., Mosby, 2021.

Solmaz, Soner et al. “Cobalamin deficiency can mask depleted body iron reserves.” Indian journal of hematology & blood transfusion : an official journal of Indian Society of Hematology and Blood Transfusion vol. 31,2 (2015): 255-8. doi:10.1007/s12288-014-0417-x