Iron

Diurnal variation
30% Biological variation
Influenced by inflammation, diet
Detected by coupled enzymatic reaction which form coloured complexes -> photometry
Not used to determine iron status, ferritin and transferrin are used instead

Ferritin

Free iron ions are toxic to the cells, as they create reactive oxygen species through the Fenton reaction. They are therefore stored in a protein called ferritin. One molecule of ferritin can store up to 4500 iron atoms. Note that the pure protein-only, non-iron-bound protein is called apoferritin. Ferritin is mostly found in the liver.

Ferritin is a positive acute phase protein.

Ferritin in laboratory medicine

Small amounts of ferritin leaks into the circulation, which can be measured in the serum. Measuring ferritin is the best way to evaluate iron status, for example in the evaluation of suspected iron deficiency. Low levels (< 15 µg/L) indicates iron deficiency, while levels between 15 and 30 indicate decreased iron stores but not overt deficiency.

However, as a positive acute phase protein, levels will be increased in case of inflammation or infection, and so can give a false negative result if these are present at the time of measurement. To evaluate iron status during acute phase reaction, one can measure transferrin receptor.

Elevated ferritin levels are usually due to acute phase reaction, but in the absence of that, high levels indicate iron overload, either due to haemochromatosis, excessive blood transfusion, or iron intoxication. Hepatitis can also cause elevated ferritin levels as ferritin is released from damaged hepatocytes. Elderly can have elevated ferritin levels up to 1000 µg/L without any underlying pathology.

Ferritin levels is measured by electrochemiluminescent immunoassay.

Transferrin

Transferrin is a serum protein whose function is to transport iron in the blood; more specifically, each molecule of transferrin binds two Fe³⁺ ions with high affinity. Transferrin binds to transferrin receptors on cells which require iron, after which the transferrin-transferrin receptor complex is endocytosed. The pure protein, non-iron-bound form of transferrin is called apoferritin. Transferrin is a negative acute phase protein.

The liver up-regulates the synthesis of transferrin in case of long-term iron deficiency.

Transferrin in laboratory medicine

Transferrin is measured clinically as a surrogate for the body's iron stores. It's not necessary to measure routinely in the evaluation of iron status, as a decreased ferritin level is sufficient for the diagnosis. There is some diurnal variation, so it should be measured in the morning.

As a negative acute phase protein, it'll be falsely low in case of infection or inflammation. Elevated levels of transferrin indicates long-term iron deficiency.

Soluble transferrin receptor

While transferrin receptor is mainly found on cell surfaces, a number of free (soluble) transferrin receptors (s-TfR) can be found in the serum as well. This is mostly from the extracellular part of the receptor of immature erythrocytes being proteolytically cleaved when the erythrocytes mature. The level of soluble transferrin receptor in the serum is proportional to the total number of transferrin receptor in tissues.

Because the soluble transferrin receptor is not an acute phase protein, its level is unaffected by the acute phase and therefore by inflammation and infection. This makes s-TfR good to evaluate iron status in patients who currently have inflammation or infection.

Elevated s-TfR levels (> 4.3 mg/L) indicate iron deficiency.

Transferrin saturation and TIBC

Transferrin can also be used to calculate transferrin saturation and total iron binding capacity (TIBC). Transferrin saturation measures how much of the serum transferrin is saturated with iron. TIBC measures how much iron the transferrin in the serum could bind at max. However, normally, practically 100% of serum transferrin is saturated with iron. Because each molecule of transferrin can bind two Fe³⁺ molecules, TIBC is calculated as 2 x serum transferrin level. Then, the transferrin saturation can be calculated as serum iron divided by TIBC.

Transferrin saturation and TIBC is not necessary to diagnose iron deficiency; ferritin alone is sufficient for that. A decreased transferrin saturation indicates iron deficiency and is less affected by acute phase than ferritin.

Both transferrin saturation and TIBC can be used in the evaluation and follow-up of haemochromatosis.

Summary

Disorder	Serum iron	Transferrin	Ferritin	Soluble transferrin receptor	Reticulocytes
Iron deficiency anaemia	↓	↑	↓	↑	↓
Haemoglobinopathy	Normal	Normal	Normal	Normal	Normal
Anaemia of chronic disease	↓	Normal/↓	Normal/↑	Normal	↓
Haemochromatosis	↑	↓	↑	↓	Normal