Anaemia
A reduced concentration of haemoglobin in the blood, limiting the amount of oxygen carried to the tissues.
What it means
Anaemia is a reduced concentration of haemoglobin — the oxygen-carrying protein inside red blood cells — below the reference range for a person's age and sex.
It is a laboratory finding rather than a diagnosis in itself. Anaemia always has a cause, and identifying that cause is the substance of the assessment.
Anaemias are conventionally classified by the average size of the red cells, measured as the MCV:
- Microcytic — small cells, typically iron deficiency or thalassaemia trait
- Normocytic — normal-sized cells, typically anaemia of chronic disease, kidney disease or acute blood loss
- Macrocytic — large cells, typically vitamin B12 or folate deficiency, hypothyroidism or alcohol excess
In practice
Reported on the full blood count, where haemoglobin, MCV and related indices appear together.
The term is qualified in clinical use — iron deficiency anaemia, pernicious anaemia, haemolytic anaemia, sickle cell anaemia — each describing a distinct mechanism.
Why it matters
Symptoms depend on severity and on how quickly the anaemia developed. Gradual anaemia may be well tolerated at levels that would cause marked breathlessness if reached suddenly. Common features include fatigue, breathlessness on exertion, pallor and palpitations.
In men of any age and in anyone over 50, iron deficiency anaemia is regarded as a possible marker of gastrointestinal blood loss and prompts investigation of the bowel rather than iron replacement alone.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026