Transferrin saturation: what it is, what it tells you and why we test it

Transferrin is the protein that carries iron in your blood. The saturation shows what share of its seats are taken by iron: in practice, how much iron is ready to use. Read with ferritin, it helps tell whether iron is short, too high or held back by inflammation.

Why we test it

Iron is needed to make red blood cells, which carry oxygen all around your body. In your blood it travels bound to transferrin, which delivers it where it is needed.

The saturation shows what share of transferrin’s seats are taken by iron. If it is low, little iron reaches the place where red cells are made. If it is high, your body may be building up too much iron.

A high saturation is the typical sign of haemochromatosis, an inherited condition. Found early, it can be treated, and damage to the liver, joints and other organs can be prevented.

What it tells you, and what it does not

If it is low, iron is often short. But it also drops with inflammation, which holds iron back in the stores even when there is enough.

If it is high, it can mean too much iron. But if you took an iron supplement in the two days before, that is usually why.

And, like the iron in your blood, it changes from one day to the next. That is why one number is not enough: it is read together with ferritin.

That is why it is read together with other tests:

Normal transferrin saturation levels

The lab decides what is normal. It is usually 20 to 45%. Your own report may print slightly different limits.

On an Italian report it may also be called IST, short for indice di saturazione della transferrina. It is the same value.

Below 20%, little iron reaches the place where red cells are made. Above 45%, a doctor usually wants to find out whether there is too much iron.

In the Rosa report you also see your optimal range, read together with ferritin, serum iron and TIBC.

What your result means

Find the row for your result, as a percentage. Which values should a doctor see? Above 45%, always: the test is usually repeated and ferritin is checked.

If your saturation is low or high

What causes high saturation

The first is the easiest to check. The others need looking into with a doctor.

High and low transferrin: what they mean

Transferrin is the protein that carries iron. It is not a separate test in the Rosa panel: many labs use it to work out TIBC, and the saturation is calculated from TIBC.

A high transferrin is usually found when iron is short, together with low ferritin and a low saturation. Then what matters is finding out why.

A low transferrin is found most often with inflammation, and more rarely with liver disease, a very poor diet or too much iron. On its own it says little: it is read with ferritin and CRP.

Low or high transferrin saturation: what to do

It depends on the cause, and ferritin helps find it. Where to start:

What can skew the result

It follows the iron in your blood, which moves easily. It helps to know what shifts it before you read it.

When to worry

Talk to your doctor if:

With Rosa, a transferrin saturation above 45% is always flagged to a doctor, who usually has it repeated fasting.

Frequently asked questions

How is transferrin saturation calculated? The iron in your blood, serum iron, is divided by TIBC, the total seats for iron. The result is given as a percentage. Many labs work out TIBC from transferrin.

Does a high saturation mean haemochromatosis? Not necessarily: it can come from an iron supplement, or from a number that changes from day to day. If it stays high, your doctor checks your ferritin and may ask for a genetic test.

Do I need to fast for a transferrin saturation test? Yes, it is better: after a meal the iron in your blood changes for a few hours, and so does the saturation. You can drink water.

What does IST mean on an Italian report? It is short for indice di saturazione della transferrina: it is the same test. Some reports print TSAT, or transferrin saturation in full.

Low saturation but normal ferritin: what does it mean? Inflammation is often behind it, holding iron back in the stores. But inflammation can also make a low ferritin look normal: that is why CRP is checked, and you talk it through with your doctor.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it once a year. If it is low and you change your diet or start treatment, it is usually rechecked after about three months, or when your doctor says.

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