Full blood count: what it measures and what your result means

What does a full blood count measure?

• Haemoglobin: the protein in red blood cells that carries oxygen. Low haemoglobin is called anaemia.
• Red blood cell count and haematocrit: how many red cells you have, and how much of your blood they make up.
• MCV (mean cell volume): the average size of your red blood cells. MCH and MCHC show how much haemoglobin they carry.
• RDW: how much your red cells vary in size.
• White blood cell count: the total, plus the five types: neutrophils, lymphocytes, monocytes, eosinophils and basophils.
• Platelets: cell fragments that help your blood clot. MPV shows their average size.

Why is a full blood count done?

• You feel tired, weak or breathless, or look pale
• You get frequent infections, or have had a fever for a while
• You bruise or bleed easily
• As part of a general health check
• You're being monitored for a condition or a medicine that affects the blood

Haemoglobin and red blood cells: what a high or low result can mean

Low haemoglobin (anaemia) is the most common finding. The size of your red cells (MCV) helps point to the cause:

• Small red cells: most often iron deficiency, or an inherited trait such as thalassaemia
• Large red cells: often low B12 or folate, alcohol, liver conditions or an underactive thyroid
• Normal-sized cells: recent blood loss, kidney disease or a long-term illness

High haemoglobin can be caused by dehydration, smoking, living at altitude or testosterone treatment. Rarely, it's caused by a bone marrow condition.

White blood cells: what a high or low result can mean

A high white cell count usually means your body is fighting an infection or inflammation. Smoking, stress, pregnancy and steroid medicines can also raise it. Very rarely, it's a sign of a blood cancer such as leukaemia.

A low white cell count is often caused by a recent viral infection, some medicines, or low B12 or folate. Some people of Black African, African-Caribbean or Middle Eastern heritage naturally have lower neutrophil counts, which is healthy.

Platelets: what a high or low result can mean

A high platelet count is usually a reaction to infection, inflammation, iron deficiency or recent surgery. A low platelet count can be caused by a viral infection, alcohol, liver conditions, some medicines, pregnancy, or the immune system attacking platelets. Rarely, either can be caused by a bone marrow condition.

What can affect your result?

• A recent infection, even a cold, can change your white cell and platelet counts for a few weeks
• Being dehydrated can raise haemoglobin
• Smoking, pregnancy and living at altitude affect red cell results
• Some medicines affect white cells and platelets
• You don't need to fast for a full blood count on its own

When to see a GP

Your results report shows the laboratory's normal range and flags any result outside it. Speak to your GP if:

• Any part of your result is flagged, especially haemoglobin, white cells or platelets
• You have symptoms of anaemia, such as tiredness, breathlessness or palpitations
• You get frequent infections

See a GP promptly if you have unexplained bruising or bleeding, night sweats, weight loss or swollen glands. Get urgent help for heavy bleeding that won't stop, or a fever with a rash that doesn't fade when pressed.