Therapeutic venesection, or phlebotomy, is the standard treatment for iron overload in hereditary haemochromatosis.
The principle is simple: remove blood, and you remove iron.
Understanding the basic arithmetic helps explain why genuine iron overload usually takes many venesections to correct, and why a very rapid fall in ferritin after only a few venesections may suggest that the original ferritin elevation was not due entirely to excess iron.
How much iron is normally in the body?
A healthy adult male contains approximately 3–5 grams of iron in total.
Most of this iron is contained in haemoglobin inside red blood cells. Smaller amounts are stored in the liver and other tissues, mainly in the form of ferritin and haemosiderin.
The body is very efficient at conserving iron. Under normal circumstances, only a small amount of iron is lost each day, and the body has no effective mechanism for actively excreting large quantities of excess iron.
This is why iron can gradually accumulate over many years in hereditary haemochromatosis.
How much iron does one venesection remove?
A standard therapeutic venesection usually removes around 500 mL of blood.
That amount of blood contains approximately 200–250 mg of elemental iron, predominantly within haemoglobin.
For practical purposes, a useful rule is:
One 500 mL venesection removes approximately 250 mg of iron.
After blood is removed, the bone marrow has to produce new red blood cells. To make new haemoglobin, the body draws iron from its stored reserves.
Repeated venesections therefore gradually reduce excess iron stores.
The arithmetic of iron removal
The numbers are quite straightforward.
If one venesection removes approximately 250 mg of iron:
4 venesections remove about 1 gram of iron
20 venesections remove about 5 grams
30 venesections remove about 7.5 grams
40 venesections remove about 10 grams
This is useful when trying to conceptualise the iron burden in significant haemochromatosis.
A normal adult man may contain approximately 3–5 g of iron in total. In significant haemochromatosis, the total body iron burden may rise to around 10–15 g or more.
Importantly, the aim of treatment is not to remove all of that iron.
The patient still needs a normal amount of iron for haemoglobin production and normal cellular function. The aim is to remove the excess iron, while leaving a normal physiological iron pool.
For example, if a patient has a total body iron burden of approximately 10–15 g and we want to return them towards a normal body iron content of roughly 3–5 g, we may need to remove approximately 5–10 g of excess iron.
At 250 mg per venesection, this corresponds roughly to:
5 g excess iron = about 20 venesections
7.5 g excess iron = about 30 venesections
10 g excess iron = about 40 venesections
This explains why patients with substantial iron overload often require weekly or regular venesections for many months before the excess iron has been adequately removed.
What happens to ferritin during venesection?
Ferritin is commonly used to monitor iron stores.
In uncomplicated hereditary haemochromatosis, ferritin generally falls as excess iron is progressively removed.
However, ferritin is not simply a direct measurement of how many grams of iron are stored in the body.
Ferritin is also affected by a number of other conditions, including:
- fatty liver disease
- obesity and metabolic syndrome
- alcohol-related liver injury
- inflammation
- infection
- malignancy
- other forms of liver injury
This is why a high ferritin does not automatically mean that the patient has severe iron overload.
Two patients can have the same ferritin level but very different amounts of stored iron.
Why the speed of ferritin reduction can be informative
The response to venesection can sometimes provide an important clue about how much of the original ferritin elevation was actually related to iron overload.
Consider a patient with a ferritin of 1,500–2,000 µg/L.
If the patient truly has many grams of excess stored iron, a substantial amount of iron has to be physically removed before those stores are depleted.
After four venesections, only about:
4 × 250 mg = 1 gram of iron
has actually left the body.
If ferritin falls dramatically after only four venesections, for example from 2,000 µg/L to near-normal levels, that does not prove that there was no iron overload.
However, it does suggest that a substantial component of the original ferritin elevation may have been driven by factors other than stored iron.
Possible contributors include metabolic fatty liver disease, alcohol-related liver injury or inflammation.
In other words, if only about 1 g of iron has been removed, yet ferritin has fallen by a very large amount, it becomes less plausible that the original ferritin elevation represented 5–10 g of excess iron by itself.
That is a useful clinical clue.
Why this distinction matters
This is especially relevant in patients with metabolic syndrome or fatty liver disease.
These patients may have markedly elevated ferritin levels without having correspondingly severe iron overload.
Ferritin may be elevated because of liver injury, inflammation and altered iron handling associated with metabolic disease.
In such cases, venesection may produce a relatively rapid initial fall in ferritin because part of the ferritin elevation was not reflecting large quantities of stored iron in the first place.
By contrast, a patient with genuine substantial iron overload may require dozens of venesections before iron stores are depleted.
Ferritin is useful, but it is not an iron meter
Ferritin is an excellent clinical marker, but it has to be interpreted in context.
Assessment of suspected iron overload should also take into account:
- transferrin saturation
- HFE genotype
- liver function
- alcohol intake
- metabolic risk factors
- inflammatory markers
- and, where appropriate, MRI measurement of liver iron concentration
MRI liver iron quantification is particularly useful when it is unclear whether a markedly elevated ferritin represents true iron overload or predominantly metabolic or inflammatory hyperferritinaemia.
A simple way to think about venesection
The easiest way to conceptualise treatment is as controlled iron subtraction.
500 mL blood ≈ 250 mg iron
Therefore:
4 venesections ≈ 1 g iron removed
20 venesections ≈ 5 g
30 venesections ≈ 7.5 g
40 venesections ≈ 10 g
A patient with significant haemochromatosis may have a total body iron burden of 10–15 g or more, but we do not aim to remove all of it. We remove the excess while preserving the normal physiological iron pool.
This is why meaningful iron depletion generally requires many venesections.
It also explains why a dramatic fall in ferritin after only a few venesections should make us consider whether the original ferritin elevation reflected more than iron overload alone.
The most useful question is therefore not simply:
“How high is the ferritin?”
but also:
“How much iron did we actually have to remove?”