Full blood count (FBC)

Note: this site is for informational purposes only. To view test results or book a test, use the NHS app in England or contact your GP.

This test measures the numbers and types of cells in your blood, including red blood cells, white blood cells, and platelets. It’s one of the most common blood tests and helps doctors check for conditions like anaemia, infections, and blood disorders.

Also known as 
FBC 
Formal name 
Full blood count 

Who needs this test

This is one of the most common blood tests. Your doctor may request it:

  • as part of a routine health check
  • if you feel generally unwell or tired
  • if you have signs of infection (fever, chills, body aches)
  • if you have unexplained bruising or bleeding
  • if you have symptoms of anaemia (fatigue, weakness, pale skin, breathlessness)
  • to monitor ongoing conditions like anaemia or blood disorders
  • to check how treatment is working (for example, chemotherapy or medications that affect blood cells)
  • before surgery or medical procedures

This test gives your doctor a broad picture of your general health and can help identify many different conditions.

Preparing for your test

A blood sample is taken from a vein in your arm. For babies and young children, a finger-prick or heel-prick sample may be used instead.

No special preparation is needed. You can eat and drink normally before the test.

Understanding your results

Your doctor will explain your results and what they mean for you. The test measures three main types of blood cells and provides detailed information about each.

What the test measures

Red blood cells carry oxygen around your body. The test measures:

  • how many red blood cells you have
  • how much haemoglobin (oxygen-carrying protein) is in your blood
  • the size and shape of your red blood cells

White blood cells fight infection. The test measures:

  • how many white blood cells you have
  • the different types of white blood cells (neutrophils, lymphocytes, monocytes, eosinophils, and basophils)

Platelets help your blood clot to stop bleeding. The test measures:

  • how many platelets you have
  • the average size of your platelets

What your results mean

Many different conditions can affect your blood cell counts. Your doctor will look at your results alongside your symptoms, medical history, and other test results. Some changes in blood cells are temporary and resolve on their own, while others need treatment or monitoring.

If your red blood cells are low

A low red blood cell count means you have anaemia. This can make you feel tired, weak, or breathless.

Common causes include:

  • not enough iron in your diet (iron deficiency anaemia)
  • vitamin B12 or folate deficiency
  • long-term inflammation or chronic diseases
  • heavy menstrual periods
  • bleeding in your digestive system
  • inherited conditions like thalassaemia

The size of your red blood cells helps identify the cause.

  • Small cells usually suggest iron deficiency, long-standing inflammation, or thalassaemia.
  • Large cells usually suggest vitamin B12 or folate deficiency, liver disease, underactive thyroid, or alcohol excess.

If your red blood cells are high

A high red blood cell count can happen if:

  • you’re dehydrated (the cells are more concentrated)
  • you live at high altitude (your body makes more to carry oxygen)
  • you have a lung or heart condition affecting oxygen levels
  • you smoke
  • you have a rare condition where your bone marrow makes too many red blood cells

If your white blood cells are high

A high white blood cell count usually means your body is fighting something. This could be:

  • a bacterial or viral infection
  • inflammation in your body
  • a reaction to some medications
  • stress or physical trauma
  • smoking
  • rarely, leukaemia or other blood disorders

The type of white blood cell that’s high gives more information.

  • High neutrophils: usually bacterial infection, inflammation, or stress.
  • High lymphocytes: usually viral infection, or rarely chronic lymphocytic leukaemia.
  • High monocytes: can occur with certain infections like TB, or bone marrow disorders.
  • High eosinophils: usually allergies (asthma, hay fever, eczema), drug reactions, or parasitic infections.
  • High basophils: less common, but can occur with some bone marrow disorders.

Important: the ​‘normal’ level of neutrophils varies between ethnic groups. Your doctor will take this into account when interpreting your results.

If your white blood cells are low

A low white blood cell count can happen with:

  • some viral infections
  • some medications (including chemotherapy)
  • autoimmune conditions where your body attacks its own cells
  • bone marrow problems
  • severe infections
  • enlarged spleen
  • liver disease

If your platelets are high

High platelets can occur:

  • after bleeding, surgery, or injury (temporary response)
  • with infections or inflammation
  • in people without a spleen or with an underactive spleen
  • with some bone marrow disorders
  • during pregnancy

Very high platelets may increase the risk of blood clots, though this is uncommon.

If your platelets are low

Low platelets can cause easy bruising or bleeding. This can happen with:

  • immune conditions (like ITP or lupus) where your body destroys platelets
  • vitamin B12 or folate deficiency
  • some medications (especially chemotherapy)
  • heavy alcohol use
  • liver disease
  • enlarged spleen
  • bone marrow disorders
  • some rare inherited conditions

Reference ranges

Reference ranges vary slightly between laboratories and depend on factors like age, sex, and ethnicity. Your results will show the reference range used by your laboratory.

Generally, ​‘normal’ ranges for adults are approximately as follows.

  • Red blood cells: 4.5 to 5.5 million cells per microlitre (men), 4 to 5 million cells per microlitre (women)
  • White blood cells: 4,000 to 11,000 cells per microlitre
  • Platelets: 150,000 to 400,000 cells per microlitre

Important: Babies and children have different normal ranges. Your doctor will interpret your results based on the appropriate reference range for your age.

Questions to ask your doctor

  • Which of my blood cell counts are outside the normal range?

  • What might be causing these changes?

  • Do I need any other tests to find the cause?

  • Is this likely to be temporary or ongoing?

  • Do I need treatment or will this resolve on its own?

  • When should I have another blood test?

  • Are there any dietary changes I should make?

  • Could any of my medications be affecting my results?

  • What symptoms should I watch for?

What happens next

Your doctor will discuss your results with you and agree on next steps together. What happens next depends on which blood cells are affected and your symptoms.

Important: your test results will be affected if you have recently had a blood transfusion. Your doctor will take this into account or may suggest repeating the test later.

What can affect your results

Your results may be affected by:

  • recent blood transfusions
  • certain medications (especially chemotherapy)
  • pregnancy
  • recent illness or infection

Your doctor will take these factors into account when interpreting your results.

Other tests you might need

Depending on your blood count results, your doctor may request additional tests.

About blood cells

Your blood contains three main types of cells, all made in your bone marrow (the spongy tissue inside your bones). Red blood cells carry oxygen around your body. White blood cells protect you from infection. Platelets help your blood clot to stop bleeding. Your body carefully controls the number of each type of cell.

For more information about blood tests see NHS website

NHS