What a full blood count tells you

A full blood count, or FBC, is one of the most common blood tests there is. In the US it's called a complete blood count, or CBC. It counts the three kinds of cells in your blood and measures a few things about them.

Doctors order it for routine check-ups, before operations, and when someone feels tired or unwell without an obvious reason. The report usually runs to a dozen lines or more, each with a number and a range.

The short version

Red blood cells carry oxygen. Low numbers can mean anaemia.
White blood cells fight infection. They often rise for a few days when you're ill.
Platelets help your blood clot after a cut.

A single result slightly outside the range is common, and on its own it often means little. Your doctor reads the whole report together.

Red blood cells

Red cells carry oxygen from your lungs to the rest of your body. Three lines on the report describe them: how much oxygen-carrying protein you have, how much of your blood is made of red cells, and how big those cells are.

Haemoglobin

Hb · g/dL

The protein inside red cells that carries oxygen. It's the main number doctors look at for anaemia.

If it's low: Often anaemia, from low iron, low B12 or folate, or blood loss.
If it's high: Less common. Dehydration and smoking can both push it up.

Haematocrit

Hct · L/L

The share of your blood made up of red cells. It usually moves up and down with haemoglobin.

Mean cell volume

MCV · fL

The average size of your red cells. It helps your doctor work out why haemoglobin is low.

If it's low: Small cells. The most common reason is low iron.
If it's high: Large cells. Low B12 or folate, alcohol and some medicines can cause it.

If your MCV is low and your ferritin is low too, the two usually point to the same thing: your iron stores are running down.

Read about the ferritin test

White blood cells

White cells are your immune system's working parts. The report gives a total count and then breaks it down by type. The two you'll see most often discussed are neutrophils and lymphocytes.

White cell count

WBC · ×10⁹/L

The total number of white cells in a sample of your blood.

If it's low: Can follow a viral illness, and some medicines lower it.
If it's high: Usually infection or inflammation. Steroids and stress can raise it too.

Neutrophils

×10⁹/L

The first responders, especially to bacterial infections. They're the largest share of your white cells.

Lymphocytes

×10⁹/L

The cells that deal with viruses and remember infections you've had before.

Platelets

Platelets are small cell fragments that gather at a cut and help your blood clot. A count a little outside the range is common and often settles by itself.

Platelet count

PLT · ×10⁹/L

How many platelets are in a sample of your blood.

If it's low: Can follow a viral illness or come from some medicines. Very low counts make bruising easier.
If it's high: Often a short-term response to infection, inflammation or low iron.

Questions for your doctor

If something on your report is flagged, these three questions cover most of what's worth knowing.

Is anything outside the range, and does it matter for me?
Do any of these results need repeating, and when?
Could any of my medicines affect these numbers?

The ranges on this page are typical for adults, with an example result marked on each bar. Your lab prints its own range beside each result, and that's the one to go by.