What iron studies include

Iron studies usually combine four lines: ferritin, serum iron, transferrin or total iron-binding capacity (TIBC), and transferrin saturation. The NHS notes that TIBC and transferrin measure essentially the same thing, so your results will usually show one or the other.

Ferritin is the body’s iron reserve, like a strongbox put away for later. A reserve and a flow answer different questions, and Lab Tests Online UK explains that ferritin, not serum iron, is now the recommended measure when iron deficiency is suspected, because stores fall well before the iron in your blood does.

  1. FerritinThe reserve, printed in µg/L on UK results, and the line that matters most for the common question: are you short of iron?
  2. Serum ironWhat is circulating at the moment of the sample. It varies through the day and from one day to the next, so it is never read alone.
  3. TIBC or transferrinHow much capacity the blood has to carry iron. It goes up when iron is short and is low or normal when there is too much.
  4. Transferrin saturationThe percentage of that capacity actually filled. UK gastroenterologists treat a saturation below 20% as a sign of iron shortage.

Iron studies or a full blood count: which shows low iron?

A full blood count does not measure iron. It shows your haemoglobin, in g/L on UK results, and the size of your red cells, which change only once a shortage has lasted long enough to affect red cell production. Anaemia is usually defined as a haemoglobin below 130 g/L in men and below 120 g/L in women who are not pregnant.

The first stage of iron deficiency, iron depletion, has no anaemia at all: the stores are empty while serum iron and haemoglobin can still look normal. A normal blood count therefore does not rule out low iron; ferritin is the line that answers that question.

What ferritin level counts as iron deficiency?

This is the part worth knowing before you compare your figure with anything. UK results print ferritin in µg/L, which is the same number as the ng/mL used on American results, so 30 µg/L and 30 ng/mL are identical.

The British Society of Gastroenterology gives a ladder rather than one line. A ferritin below 15 µg/L means iron stores are absent; below 30 generally means they are low; above 150, absolute iron deficiency is unlikely. When inflammation is present, it cites 45 as the best trade-off, and in chronic kidney disease the threshold rises to 100.

Other experts go further. A systematic review of 29 guidelines suggested considering a cut-off of 100 in most conditions, except in particular situations such as young healthy women with heavy periods, while a Swiss consensus found 30 accurate in people without inflammation. Read together, the practical conclusion is clear: a ferritin of 25 printed just above the laboratory’s lower limit is not automatically fine. This is a question for your GP with your symptoms in hand, not a line to settle from a table.

15
Below this, iron stores are absent (µg/L)[1]
30
Below this, stores are generally low (µg/L)[1]
45
Suggested cut-off when inflammation is present (µg/L)[1]

Inflammation can hide a deficiency

Ferritin is also an acute-phase protein: it rises with inflammation, independently of how much iron you have stored. Inflammatory bowel disease, heart failure, chronic kidney disease or a passing infection can therefore lift ferritin into its normal range while the reserve underneath is low.

This is why iron studies are read next to an inflammation marker such as CRP, and why transferrin saturation is useful. In those chronic conditions, a review of the evidence treats a ferritin below 100, or a saturation below 20%, as consistent with iron deficiency.

The reverse case is simpler: inflammation pushes ferritin up, not down, so a low ferritin is hard to explain away.

Low or high: what each pattern suggests

Low iron or low ferritin: which is worse? They are not rivals but different measurements. Every guideline in the 29-guideline review defined iron deficiency with ferritin, because it reflects the reserve. A low serum iron on its own describes a single moment, and inflammation can hold it down even when stores are adequate.

High values have their own pattern. The NHS explains that in iron overload, such as haemochromatosis, serum iron is high while TIBC is low or normal; a high ferritin on its own more often reflects inflammation than too much iron. TIBC is also low in liver disease, because the liver makes transferrin. Which explanation applies is read from the whole set, so a high value is a reason for a conversation with your GP rather than a conclusion.

Before the test, and after it

Lab Tests Online UK notes that you may be asked to fast for 12 hours, with water only, before some iron tests, and that you should not take iron tablets in the 24 hours before, because iron is absorbed quickly and can make blood levels look falsely high. If you buy a test privately, check that it includes ferritin and transferrin saturation rather than serum iron alone; what private blood tests cost is covered separately.

And if a deficiency is found, the question of why matters more than the number. The British Society of Gastroenterology estimates that, in high-income countries, iron deficiency anaemia affects 2 to 5% of adult men and postmenopausal women and 5 to 12% of premenopausal women, and that about a third of men and postmenopausal women with it have an underlying cause, most often in the gut. Coeliac disease is found in 3 to 5% of cases. In the UK, unexplained iron deficiency anaemia in people at risk is a recognised reason for fast-track referral.