Understanding Iron and Anaemia

How haemoglobin, ferritin, MCV and red cell counts relate, and why iron stores can fall before anaemia appears.

Iron problems are rarely described by a single number. A routine full blood count measures haemoglobin, the red cell count and the average size of those cells, while ferritin — a separate test that is not part of a full blood count — describes how much iron the body has in storage. Because these results often arrive on the same report, and sometimes on different reports weeks apart, it is easy to read one in isolation and miss what the group is saying.

This collection exists because those measurements are usually explained separately. Here they are placed in the order a clinician tends to read them: how much oxygen-carrying protein is present, how many cells are carrying it, how big those cells are, and how much iron is left in store. Seeing the sequence makes it clearer why a normal haemoglobin does not always mean iron levels are comfortable, and why a low result on its own does not identify a cause.

Written by Omer Suliman · Last reviewed September 2026 · Educational information only

Medipher explains medical wording in plain English — it does not diagnose or replace a clinician. If this is a medical emergency, call your local emergency number immediately.

What this collection covers

Haemoglobin
Ferritin
MCV
Red blood cells
Anaemia

How these tests relate

Why ferritin can fall before haemoglobin does

The body draws on stored iron to keep making haemoglobin. Stores can therefore run low for some time while haemoglobin still sits inside the laboratory's usual range. This is why a ferritin result is sometimes requested even when a full blood count looks unremarkable, and why the two are read together rather than one after the other.

Why cell size is a clue rather than an answer

MCV describes the average size of red blood cells. Smaller-than-usual cells are often discussed alongside iron, and larger-than-usual cells alongside other nutrients, but size is one observation among several. It narrows what a clinician considers; it does not establish a cause on its own.

Why ferritin can also rise

Ferritin behaves differently during inflammation or illness, when it can read higher than iron stores alone would suggest. That is one reason a single ferritin value is interpreted alongside the rest of the picture and the person's own circumstances, rather than read as a direct measure of iron.

What anaemia does and does not mean

Anaemia is the word used when haemoglobin sits below the usual range for a person's age and sex. It describes a finding, not a diagnosis — there are many possible reasons, of which low iron is only one. Working out which applies is a clinical judgement based on history, examination and often further tests.

Your learning pathway

Follow the steps in order, or jump to the part you need. Each step opens either a guide — our fuller explanation of a topic — or a glossary entry defining a single term. The emphasis is understanding, not diagnosis.

About this page

Author: Omer Suliman
Medical reviewer: Clinical review pending
Last reviewed: September 2026

Evidence. Written from public patient-information sources including the NHS, NICE and the Association for Laboratory Medicine, and reviewed for plain-English accuracy before publication.

Educational content only. Medipher explains medical language in plain English. It does not diagnose, and it is not a substitute for advice from a doctor, nurse or pharmacist. Read the medical disclaimer.