Prepared by Gunnar Nordin
Discussed and decided by the Committee/Subcommittee on Nomenclature for Properties and Units 2025-12-17.
Summary
Total iron binding capacity (TIBC) refers to the maximum amount of iron that plasma proteins can bind. This quantity can be estimated by chemical methods or calculated from the estimation of the substance concentration of transferrin. The NPU terminology has so far not distinguished this as two different properties. With the changes introduced May 1st 2026 the NPU terminology recognizes the two properties with separate NPU codes and definitions. As a consequence, saturation fractions for iron are also distinguished as different properties depending on how the total iron binding capacity has been estimated. In summary two new NPU codes have been created, and the description of 5 NPU codes have been modified.
Background
The concept “total iron binding capacity” and the component “iron binding capacity” in the current NPU terminology has been challenged which initiated a discussion about what exactly is meant by “iron binding capacity” in e.g.:
NPU04133 Plasma—Iron binding capacity; substance concentration = ? µmol/L
A Component in the NPU terminology is a physical entity that can be a substance (e.g. sodium ion, proteins) or a process (e.g. coagulation). “Iron binding capacity” is not a physical entity, rather it refers to a property of iron-binding proteins. Hence, it is unclear what “iron binding capacity” is.
Free iron ions (Fe3+) are toxic and cannot be found with regular methods in normal plasma. Fe3+are bound to proteins in plasma, mainly transferrin. Chemical methods can be used to measure the maximal amount of Fe3+ that possibly can be bound to plasma proteins at the saturation limit. This amount can be termed “total iron binding capacity estimated by chemical means” (TIBCchem).
The amount of Fe3+, needed in addition to what is already in a sample, to reach full saturation of the binding proteins can also be measured and is often termed “unbound iron binding capacity” (UIBC). TIBCchem can thus be calculated as the sum of UIBC and the original amount of Fe3+ in plasma.
Measurement of transferrin is another principle to estimate iron binding capacity under the assumption that each transferrin molecule binds 2 molecules of Fe3+ at saturation (Baker 1992). This quantity can be termed “total iron binding capacity estimated as twice the amount of transferrin” (TIBCtrf). Although the quantities intended to be measured are similar, TIBCchem and TIBCtrf are not exactly the same.
According to the theoretical model iron will not bind to any other proteins than transferrin, and transferrin will be 100% saturated before any non-transferrin bound iron appears in plasma. Neither is exactly true. Non-transferrin bound iron and labile plasma iron start to appear already at a transferrin saturation level of 80 – 90% (de Swart et al, 2016). Iron can also to some extent bind to other proteins than transferrin, for example to ferritin. In extreme situations the TIBCchem can be higher TIBCtrf. It is also well known that TIBCchem varies with the method used (Brissot et al 2020, Mahant et al 2023) in lack of an agreed reference method. After the standardization of immunochemical measurement of proteins, including transferrin, by the reference material CRM470/IFCC, measurement of TIBCtrf can be made traceable to the SI.
TIBCtrf is often recommended because of better standardization, but TIBCchem is still in use, either for budget reasons or for use in populations with a high prevalence of genetic variants of transferrin for which TIBCtrf might be misleading. These two distinct quantities have so far not been reflected in the NPU terminology, where a single concept has been used for TIBC, that is NPU04133. Therefore, the following five changes are introduced (reworded and new NPU concepts in bold).
- NPU04133 reworded
Under the assumption that the current property
NPU04133 Plasma—Iron binding capacity; substance concentration = ? µmol/L
describes TIBCchem the full description is changed for clarity to:
NPU04133 Plasma—Iron; substance concentration(protein bound at saturation) = ? µmol/L
The verbal expression for the concept can be “the substance concentration of iron bound to protein at saturation in Mr Smiths plasma is ? µmol/L”.
This syntax is analogous to the description of the p50 value for oxygen tension (NPU03010). A procedure specification is added to indicate the need to consider the existing variations among methods.
- NPU 04132 reworded
The current description of UIBC include the same component “iron binding capacity” as NPU04133 and therefore the same clarification as above is needed. In addition, the current definition is not intuitive to understand.
NPU04132 P—Iron binding capacity(Fe;free); subst.c. = ? µmol/L
An improved description for the same concept (the same NPU code) is:
NPU04132 Plasma—Iron; substance concentration(protein bound at saturation – protein bound actual) = ? µmol/L
Note that by this definition the quantity has results on a differential scale. This is correct as, in principle, UIBC can have negative values in cases of iron overload
- New NPU code for the concept TIBCtrf
A new concept is created for the iron binding capacity as estimated from the substance concentration of transferrin (TIBCtrf) as:
NPU63513 Plasma—Transferrin(Iron binding sites); substance concentration = ? micromole per litre
The reference material is defined for the mass concentration of transferrin. For the expression of substance concentration of transferrin a molar mass of 79 570 Da can be assumed (Vernet 1993) and that each transferrin molecule can bind 2 iron molecules (Baker 1992).
- New NPU code for saturation fraction calculated as Fe/TIBCchem
Related to TIBCtrf and TIBCchem are the corresponding fractions of bound iron ions. The current description of saturation of transferrin with iron (TSAT) is
NPU04191 Transferrin(Iron binding sites;Plasma)—Iron; substance fraction = ?).
Transferrin saturation (TSAT) and saturation calculated as Fe/TIBCchem give different results and might differ depending on clinical condition (Beilby et al 1992, Gottschalk et al 2000, Eleftheridas et al 2010). A new concept corresponding to TSAT for estimation of iron saturation of TIBCchem is therefore needed:
NPU63514 Iron(protein bound at saturation;Plasma)—Iron; substance fraction = ?
- For consistency some other NPU codes need to be reworded.
For consistency the components and specifications for the following three NPU concepts, in use for oral iron absorption tests, need to be updated or replaced with new concepts:
NPU53167 P—Iron binding capacity; subst.c.(0 min) = ? µmol/L
NPU53167 P—Iron; subst.c.(0 min, protein bound at saturation, procedure) = ? µmol/L
NPU53168 P—Iron binding capacity; subst.c.(60 min) = ? µmol/L
NPU53168 P—Iron; subst.c.(60 min, protein bound at saturation, procedure) = ? µmol/L
NPU53169 P—Iron binding capacity; subst.c.(120 min) = ? µmol/L
NPU53169 P—Iron; subst.c.(120 min, protein bound at saturation, procedure) = ? µmol/L
Alternative kind of quantity and unit
For TIBCchem, and TIBCtrf it would be possible to, on request, create the corresponding NPU concepts for mass concentrations with the unit µg/L (but µg/dL is not allowed).
The quantities “TIBCchem sat” and “TIBCtrf sat” are substance fractions expressed with the unit 1 in the NPU terminology. However, the unit “%” is commonly used in the health care systems for all kind of saturations. The adoption of the unit % for “saturations” and “fractions” in the NPU terminology is discussed in a separate paper.
Conversion between TIBCchem and TIBCtrf values is not allowed because, as described above, these concepts are different.
References
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Beilby J, Olynyk J, Ching S, Prins A, Swanson N, Reed W, et al. Transferrin index: an alternative method for calculating the iron saturation of transferrin. Clin Chem. 1992;38(10):2078–81.
Brissot P, Brissot E, Loréal O, Ropert M. Laboratory medicine and iron overload: diagnostic and therapeutic aspects. J Lab Precis Med. 2020;5.
de Swart L, Hendriks JC, van der Vorm LN, Cabantchik ZI, Evans PJ, Hod EA, et al. Second international round robin for the quantification of serum non-transferrin-bound iron and labile plasma iron in patients with iron-overload disorders. Haematologica. 2016;101(1):38–45.
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Garbowski MW, Bansal S, Porter JB, Mori C, Burckhardt S, Hider RC. Intravenous iron preparations transiently generate non-transferrin-bound iron from two proposed pathways. Haematologica. 2021;106(11):2885–96.
Gottschalk R, Wigand R, Dietrich CF, Oremek G, Liebisch F, Hoelzer D, et al. Total iron-binding capacity and serum transferrin determination under the influence of several clinical conditions. Clin Chim Acta. 2000;293(1-2):127–38.
Mahant H, Jain S, Patel A, Lapani B. Appropriate Method of TIBC Estimation in Reference to Serum Transferrin Levels. J Lab Physicians. 2023;15(1):25–30.
Pfeiffer CM, Looker AC. Laboratory methodologies for indicators of iron status: strengths, limitations, and analytical challenges. Am J Clin Nutr. 2017;106(Suppl 6):1606S–14S.
Vernet M. Immunochemical assay of transferrin and iron saturation in serum. Clin Chem. 1993;39(11 Pt 1):2352–3.