Unsaturated Iron-Binding Capacity (UIBC)
How much of your blood's iron-carrying capacity is still empty. Added to serum iron, it gives total iron-binding capacity (TIBC).
AI-verified against cited sources. Awaiting expert reviewAlso called UIBCLOINC 2501-5
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The usual range for UIBC depends on sex. Choose female or male above to see where you stand.
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What it is
Iron travels in your blood attached to a protein called transferrin. UIBC stands for unsaturated iron-binding capacity. It is the part of transferrin that is not holding iron right now, like empty seats on a bus. Add serum iron and UIBC together and you get total iron-binding capacity (TIBC). It is usually reported with serum iron and TIBC, and together they give transferrin saturation.
Why it matters
UIBC is mostly a building block for other iron numbers. When iron stores run low, the body makes more transferrin and more of it sits empty, so UIBC tends to go up. When iron levels are high, more of the transferrin is full, so UIBC tends to be low. It doesn't point to a cause on its own, so it is read with serum iron, transferrin saturation and ferritin.
If it’s low
A low UIBC means little of your iron carrier protein is empty. This can happen when the iron in your blood is high, or when there is less transferrin overall, as with inflammation, liver disease or low protein levels.
Common causes
- Iron supplements or iron treatment shortly before the blood draw
- Inflammation, infection or long-term illness
- Liver disease or low protein levels
- Hemolytic anemia, where red blood cells break down early
- Iron overload conditions, including the inherited condition hemochromatosis
What can help
- Tell your clinician about any iron supplements or recent transfusions so the result can be put in context
- Ask your clinician to read it together with serum iron, transferrin saturation and ferritin
If it’s high
A high UIBC means more of your iron carrier protein is empty than usual. It most often fits with low iron stores. It can also be higher during pregnancy or with estrogen-containing birth control, where it does not mean low iron.
Common causes
- Iron deficiency (low iron stores)
- Pregnancy, especially late pregnancy
- Estrogen-containing birth control pills
- Heavy periods or other ongoing blood loss
- Not enough iron in the diet
What can help
- Ask your clinician to read it together with ferritin and transferrin saturation
- If iron stores are low, ask about the cause and whether iron treatment is right for you
- Eat iron-rich foods such as meat, fish, beans, lentils and fortified cereals
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
The usual range depends on sex and age. Choose women or men to see the one that applies.
See UIBC by age and sex, with the U.S. survey numbers where they exist.
What can affect your result
- Pregnancy and estrogen-containing birth control raise the iron-binding capacity
- Iron supplements, iron treatment or a blood transfusion shortly before the draw
- Inflammation, liver disease and low protein levels lower the amount of transferrin
- Age and sex: Labcorp's intervals differ for children and, in some age groups, between males and females
- Tube type: EDTA plasma gives decreased results
- Method: UIBC-based TIBC can differ from other TIBC methods
- Iron supplements taken shortly before the draw can change the result. Mayo Clinic Laboratories asks people not to take iron-containing supplements for 24 hours before an iron and TIBC draw.
- Labcorp asks that blood be drawn before iron treatment or a transfusion, and that iron testing wait at least four days after a transfusion.
- EDTA plasma gives decreased results, and Labcorp rejects hemolyzed or clotted samples. Labcorp accepts serum or lithium-heparin plasma.
- Estrogen-containing birth control and pregnancy raise TIBC and transferrin (Labcorp), which also changes UIBC.
- Inflammation, infection, liver disease and low protein levels lower transferrin, and with it the binding capacity.
Morning, because it is usually drawn with serum iron, which tends to be higher in the morning. Longer-term shifts follow iron stores over weeks. Day to day it can move opposite to serum iron, which varies through the day.
When to talk to a clinician
A UIBC result on its own is rarely urgent. Talk with a clinician if it is outside your lab's range together with tiredness, shortness of breath, dizziness, or serum iron, ferritin or blood counts outside your lab's ranges.
Questions you could ask
- How do my iron, UIBC, TIBC and transferrin saturation fit together?
- Could pregnancy, birth control or low iron explain a high UIBC?
- Should we check ferritin and transferrin saturation too?
- Could iron supplements, a transfusion or a high iron level explain a low UIBC?
- Should we look at ferritin and transferrin saturation together?
Questions people ask
What is the difference between UIBC, TIBC and transferrin?
Transferrin is the carrier protein. TIBC is the most iron it could carry in total, and UIBC is the part that is not carrying iron right now. TIBC equals serum iron plus UIBC.
Why is my UIBC high?
UIBC goes up when the body makes more transferrin, as with low iron stores, pregnancy, and estrogen-containing birth control. Labcorp notes that with birth control, binding capacity is higher but saturation stays normal. Ferritin and transferrin saturation help sort out which it is.
How do I convert UIBC units?
Multiply µg/dL by 0.179 to get µmol/L. To go the other way, multiply µmol/L by 5.585. For example, 200 µg/dL is about 35.8 µmol/L.
Show the science and sources
Units
- 1 µg/dL = 1 µg/dL
- 1 µg/dL = 0.1791 µmol/L
For clinicians and researchers
UIBC is the iron-binding capacity of transferrin that is not occupied by iron. TIBC equals serum iron plus UIBC, and transferrin saturation equals serum iron divided by TIBC times 100. Labcorp lists a colorimetric method for its iron and TIBC panel (test 001321), which reports iron, UIBC, TIBC and saturation; on its sample report TIBC 286 equals iron 74 plus UIBC 212 µg/dL. ARUP (test 0020420) and Mayo Clinic Laboratories (test SFEC) report iron, TIBC and saturation but showed no UIBC interval in the pages reviewed. Labcorp intervals in µg/dL: 31 days to 6 months 127–340 for both sexes; 7 months to 17 years, males 148–395 and females 131–425; 18 to 60 years, males 111–343 and females 131–425; none established for 0 to 30 days. Labcorp rejects EDTA or oxalate plasma (EDTA gives decreased results) and hemolyzed or clotted samples, and says iron testing should wait at least four days after a blood transfusion. In a 2022 comparison of about 250 samples, mean TIBC was about 344.5 µg/dL by the calculated method (iron plus UIBC), 342 by an indirect method and 378 by a direct method, so UIBC-derived values are not interchangeable with every other TIBC method. UIBC rises in iron deficiency, with oral contraceptives and in pregnancy, and falls when saturation is high or transferrin is reduced by hypoproteinemia, inflammation or liver disease.
LOINC 2501-5 (primary), 22753-8, 35216-1. Specimen: serum-or-plasma.
Sources
- Iron and Total Iron-binding Capacity (TIBC) (test 001321), Labcorp
- Iron and TIBC sample report (test 001321), Labcorp
- Pediatric Testing Reference Ranges, Labcorp (2022)
- Iron and Iron Binding Capacity (test 0020420), ARUP Laboratories
- Iron and Total Iron-Binding Capacity, Serum (test SFEC), Mayo Clinic Laboratories
- Iron Tests, MedlinePlus (NIH)
- Appropriate Method of TIBC Estimation in Reference to Serum Transferrin Levels, Journal of Laboratory Physicians (2022)
- Iron (compound summary, CID 23925), PubChem (NIH)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.