In short
- Serum iron, ferritin and transferrin each measure something different.
- Ferritin says something about your stores and falls first when a deficiency is developing.
- With inflammation, ferritin can be raised and mask a deficiency.
- An iron deficiency is a finding: the question of where the iron is going belongs with it.
Three ways of looking at iron
The first is serum iron, often simply called iron. This is the iron circulating in your blood at the moment of the draw, on its way to the bone marrow and other tissues. It sounds like the most logical test, but it is the least stable of the three.
Serum iron follows a daily rhythm and is on average higher in the morning than in the evening. It rises after an iron tablet or a meal rich in iron and falls during an infection. Two measurements in the same week can therefore differ considerably without anything having changed in your iron balance.
The second is ferritin. Ferritin is the protein your body stores iron in, mainly in the liver, spleen and bone marrow. Under normal circumstances the amount of ferritin in your blood tracks the size of those stores.
That makes ferritin the test that says something about your reserves rather than about this moment. If the store is slowly running down, for example through heavy periods or a diet low in well-absorbed iron, ferritin falls well before anything shows up in your blood count.
The third way is about transport. Transferrin is the protein that carries iron through the blood, and the iron binding capacity indicates how much iron could be bound in total.
More informative than either on its own is the transferrin saturation, calculated from your iron and the binding capacity. It shows what proportion of the available transport capacity is actually filled, in other words how much iron is available for making red blood cells. In a deficiency the body produces more transport protein while there is less iron to load onto it, so the saturation falls.
| Test | What it measures | Most useful for |
|---|---|---|
| Serum iron | the iron circulating at that moment | assessment alongside the other values |
| Ferritin | the size of your iron stores | spotting a deficiency early and following supplementation |
| Transferrin and binding capacity | how much iron can be transported | calculating the saturation |
| Transferrin saturation | what proportion of transport capacity is filled | doubt with inflammation or suspected overload |
Why ferritin is usually the first choice
An iron deficiency develops in stages:
- 1The stores run down.
- 2The production of red blood cells comes under pressure.
- 3Haemoglobin falls and anaemia appears.
Ferritin moves first in that sequence, and that is where the value of this test lies: you see a deficiency coming in the stage where it is still easy to correct. It also explains why people can have complaints while their blood count is still normal.
Complaints that can fit low stores:
- persistent fatigue
- becoming short of breath more quickly during exertion
- trouble concentrating
- more hair loss than you are used to
- cold hands and feet
- restless legs at night
Every one of those complaints is non-specific, so only a measurement can give certainty.
Have ferritin measured at Bloodworks
Ferritin as an individual test, all-in including the blood draw, doctor interpretation and a personal PDF report. No referral required, results typically within 2 working days.
When ferritin gives a distorted picture
Besides being a storage protein, ferritin is also what is known as an acute phase protein. That means the value rises as soon as there is inflammation or tissue damage somewhere in the body, regardless of how much iron you actually have in store. A recent infection, a chronic inflammatory condition such as rheumatoid arthritis or inflammatory bowel disease, liver conditions, considerable excess weight and regular alcohol use can all push ferritin up.
The consequence is deceptive: someone with a genuine iron deficiency and inflammation can have a ferritin that looks reassuring on paper. That is why ferritin is assessed alongside an inflammation marker such as CRP when in doubt, and why the transferrin saturation helps to see through it.
The other way round, a high ferritin is not immediately a reason to worry about iron overload. In most cases inflammation, alcohol use, fatty liver or excess weight is the explanation rather than too much iron.
To assess genuine overload, as in the hereditary condition haemochromatosis, a doctor looks mainly at the transferrin saturation in combination with ferritin, and weighs up your family history and liver values. If ferritin remains high on repeat testing without a clear explanation, that belongs with your GP.
Which test, and when?
For most questions about fatigue or hair loss, and for people at increased risk of a deficiency, ferritin is the logical starting point, preferably alongside a blood count. Think of heavy periods, pregnancy, a vegetarian or vegan diet, plenty of endurance training, blood donation or a gastrointestinal condition.
A full iron panel with serum iron, transferrin, binding capacity and saturation is more useful when the picture does not add up:
- complaints persist while ferritin looks reassuring
- inflammation is in play
- ferritin is high and the question is why
- you want to distinguish a genuine deficiency from a distribution problem in chronic illness
If you want to follow the effect of iron supplementation, ferritin after a few months is the most useful value, because it shows whether the store has actually been replenished.
When a blood test is not the answer
An iron deficiency is a finding, not a diagnosis. The key question is always where the iron is going. If you see blood in your stools or your stools are black, if you are losing weight unintentionally, or if you have a changed bowel pattern or abdominal complaints, see your GP, even if you already have a result.
In men and in women after the menopause, an iron deficiency without a clear explanation is a reason to investigate blood loss from the gastrointestinal tract, and a blood test cannot cover that.
Testing also makes little sense shortly after an iron infusion or a transfusion, or soon after starting iron tablets: the result then says more about the treatment than about your stores. Finally, starting iron on your own initiative without measuring is not a good idea, because too much iron causes complaints and is genuinely harmful for some people.
Having your iron status tested at Bloodworks
At Bloodworks you have your iron status tested without a referral, at one of the more than 750 affiliated collection points across the Netherlands. You can have ferritin measured on its own, or choose the full Iron panel with ferritin, serum iron, transferrin, iron binding capacity and transferrin saturation in a single draw.
A doctor assesses the values together rather than as separate numbers, and explains in your personal PDF report what they mean in your situation. You receive the result typically within 2 working days, including advice on whether a repeat test or a discussion with your GP is appropriate.
Frequently asked questions
Do I need to fast for an iron test?
For ferritin, fasting is not needed. If serum iron or a full panel is measured, a morning draw is preferable, because serum iron follows a daily rhythm and can rise after eating.
Should I pause my iron tablets before the test?
In any case, do not take an iron tablet on the morning of the draw, as this can briefly raise serum iron considerably. If you want to know how your stores stand without supplementation, discuss with a doctor how long beforehand to stop.
Can my ferritin be normal while I still have a deficiency?
Yes. With inflammation, an infection, a liver condition or considerable excess weight, ferritin can be artificially raised. That is why an inflammation marker is assessed alongside it when in doubt, and why the transferrin saturation is taken into account.
What is the difference between iron deficiency and anaemia?
An iron deficiency means the stores are falling short. Anaemia means haemoglobin has dropped, which is a later stage. So you can have an iron deficiency with complaints while your blood count is still normal.
How soon after starting supplementation can I test again?
Replenishing the stores takes time. A repeat test after around three months usually gives the most useful picture, while testing earlier says more about the tablets than about your reserves.