Transferrin Saturation

What transferrin saturation is

Transferrin saturation indicates how much of the blood's iron transport capacity is occupied. Transferrin is the protein that carries iron absorbed in the intestine or released by macrophages to the bone marrow and tissues; total iron-binding capacity (TIBC) measures how much iron it can carry.

In the review by Grotto (2010), published in the Revista Brasileira de Hematologia e Hemoterapia, transferrin normally circulates with about one third of its sites occupied. TIBC rises in iron deficiency and pregnancy, and falls in inflammation and hemochromatosis; therefore, saturation changes in both directions and helps separate these conditions.

Saturation is read together with the complete blood count. The red cell indices (MCV, MCH and MCHC) show whether the red cells are small and pale, and the Mentzer index is used as a screen between iron deficiency and thalassemia trait.

How to calculate transferrin saturation: inputs and units

The calculator asks for two tests from the iron panel, in the same unit:

• Serum iron, in µg/dL, from 0 to 1,000. Example: 80.
• Total iron-binding capacity (TIBC), in µg/dL, greater than 0 and up to 1,000. Example: 300.

Step by step:

1. Divide serum iron by TIBC.
2. Multiply by 100 to obtain the percentage.
3. The tool shows the result with one decimal place and the range according to published cutoffs, explained in the interpretation section.

Values outside the ranges, or iron greater than TIBC, produce a message saying what to correct.

Some laboratories report unsaturated iron-binding capacity (UIBC) rather than TIBC. Grotto (2010) explains that TIBC is the sum of UIBC and serum iron. In that case, add the two before entering: with iron of 80 and UIBC of 220 µg/dL, TIBC is 300 µg/dL.

Since saturation is a ratio, the result is the same in µg/dL or µmol/L, as long as both tests are in the same unit.

Transferrin saturation formula and worked example

The tool uses the relationship between serum iron and TIBC described by Grotto (2010) and by the AASLD guideline (2011). The expressions are:

Transferrin saturation (%) = Serum iron ÷ TIBC × 100
TIBC = UIBC + Serum iron

• Serum iron: iron bound to transferrin, in µg/dL.
• TIBC: total iron-binding capacity, in µg/dL.
• UIBC: unsaturated capacity, the part of transferrin still free.

The example is didactic. A saturation between the cutoffs does not exclude early iron deficiency or overload, which depend on ferritin and other data.

How to understand the transferrin saturation result

The calculator reads saturation against two cutoffs with a published source, applied to the value already rounded to one decimal place.

Three ranges are shown:

• Below 16%: the WHO guide on iron deficiency anemia (2001) describes that people with more marked anemia, who respond to iron, usually have saturation below 16%. Grotto (2010) uses the same limit as a sign of insufficient iron supply for red cell production.
• 45% or more: the AASLD guideline on hemochromatosis (Bacon and colleagues, 2011) recommends HFE gene testing when saturation is 45% or more or ferritin is above normal, in people with suggestive symptoms, findings or family history.
• From 16% to less than 45%: between the two cutoffs. The tool does not call this range normal, because it does not exclude early deficiency or overload.

The European EASL guideline (2022) uses similar limits: saturation above 45% in women, or above 50% in men and postmenopausal women, together with elevated ferritin, in HFE p.Cys282Tyr homozygotes. The WHO notes that saturation is less reliable than ferritin for detecting deficiency, because of the daily variation in serum iron. The valid reference interval is the laboratory's, and interpretation, which combines saturation, ferritin, blood count and clinical picture, is the responsibility of the professional.

Common errors, units and verification checklist

The most common error is entering UIBC in the TIBC field. With iron of 80 and UIBC of 220 µg/dL, the tool would calculate 36.4%, instead of the correct 26.7% with a TIBC of 300 µg/dL. An error message appears only when iron exceeds the value entered.

• TIBC, not UIBC: if the report gives UIBC, add the serum iron.
• Same unit: iron and TIBC both in µg/dL or both in µmol/L. To convert iron from µg/dL to µmol/L, multiply by 0.179 (molar mass of iron 55.85 g/mol).
• Same sample: iron and TIBC from the same collection.
• Collection time: Grotto describes a circadian rhythm of serum iron, higher in the morning and lower at night; laboratories usually standardize morning collection.
• Inflammation: iron and TIBC fall together, and saturation loses specificity for deficiency.
• Pregnancy and oral contraceptives: raise TIBC, according to the same review.

To assess the bone marrow response, see the reticulocyte production index.

Limitations and comparison with ferritin and other iron markers

Transferrin saturation alone does not define iron status. It depends on serum iron, which fluctuates throughout the day and falls in inflammation, and on TIBC, which changes with pregnancy, inflammation and liver disease.

The World Health Organization published in 2020 a specific guideline on the use of ferritin to assess iron status in individuals and populations, which shows the central role of this marker. In overload, the EASL guideline combines saturation and ferritin with HFE genetics and, when necessary, liver MRI. The decision about which tests to order belongs to the professional caring for the patient.

Frequently asked questions about transferrin saturation

References

1. World Health Organization. Iron deficiency anaemia: assessment, prevention and control. A guide for programme managers. Geneva: WHO; 2001 (WHO/NHD/01.3). View
2. Bacon BR, Adams PC, Kowdley KV, Powell LW, Tavill AS. Diagnosis and management of hemochromatosis: 2011 practice guideline by the American Association for the Study of Liver Diseases. Hepatology. 2011;54(1):328-43. doi:10.1002/hep.24330. View
3. Grotto HZW. Diagnóstico laboratorial da deficiência de ferro. Rev Bras Hematol Hemoter. 2010;32(Supl 2):22-8. doi:10.1590/S1516-84842010005000046. View
4. European Association for the Study of the Liver. EASL Clinical Practice Guidelines on haemochromatosis. J Hepatol. 2022;77(2):479-502. doi:10.1016/j.jhep.2022.03.033. View
5. World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations. Geneva: WHO; 2020. View