Full blood count (FBC)

You do not need to do anything special before this test. A healthcare professional will take a blood sample from a vein in your arm. In young babies, the sample may be taken from the heel. 

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.

Your blood contains three main types of cell, each with a different job: 

• Red blood cells carry oxygen around your body using a protein called haemoglobin. 
• White blood cells protect you from infection and are part of your immune system. 
• Platelets help your blood to clot when you have a cut or injury. 

An FBC measures several things about each of these cell types. The results help your doctor spot abnormalities, such as too few or too many of a particular cell, which can point to a wide range of conditions.

These measurements tell your doctor about the number, size, and quality of your red blood cells. 

This counts the number of red blood cells in your blood. Red blood cells carry oxygen to your tissues and remove carbon dioxide. 

• If your RBC count is low, this can indicate anaemia. Your doctor will look at other measurements to find out why. 
• If your RBC count is high, this can occur with dehydration, burns, or a condition called polycythaemia. 

Haemoglobin is the protein inside red blood cells that carries oxygen. This result usually mirrors your RBC count. 

• A low haemoglobin level means you have anaemia. This can make you feel tired, breathless, or weak. 
• A high haemoglobin level can occur with dehydration, smoking, or certain bone marrow conditions such as polycythaemia. 

Haematocrit measures what proportion of your blood is made up of red blood cells. It is expressed as a percentage. This result also mirrors your RBC count and haemoglobin. 

MCV measures the average size of your red blood cells. This helps your doctor understand the possible cause of anaemia. 

• If your MCV is high, your red blood cells are larger than normal. This is called macrocytic anaemia. Common causes include vitamin B12 deficiency, folate deficiency, liver disease, an underactive thyroid, alcohol excess, and some bone marrow disorders. 
• If your MCV is low, your red blood cells are smaller than normal. This is called microcytic anaemia. Common causes include iron deficiency, long-standing inflammation, and thalassaemia. 

MCH measures the average amount of haemoglobin in each red blood cell. 

• A low MCH can indicate iron deficiency anaemia, inflammatory conditions, or thalassaemia. 
• A high MCH is usually associated with macrocytic anaemia (large red blood cells). 

MCHC measures the concentration of haemoglobin within red blood cells. It helps your doctor interpret MCH results and diagnose specific red cell disorders. 

• A low MCHC is seen in iron deficiency anaemia, long-standing inflammation, and thalassaemia. 
• A high MCHC is seen in conditions such as hereditary spherocytosis, where the red blood cells are an unusual shape. 

RDW measures how much variation there is in the size of your red blood cells. 

• A high RDW means your red blood cells vary more in size than usual. This can be a sign of iron deficiency, vitamin B12 deficiency, folate deficiency, or bone marrow disorders. 

These measurements show how many white blood cells you have and what types are present. A separate test called the white blood cell differential breaks down the count into five types. These are reported together with the FBC. 

This counts the total number of white blood cells. White blood cells protect you from infection and disease. 

• A high WBC count can indicate infection, inflammation, or leukaemia and other blood disorders. 
• A low WBC count can occur with certain medicines (especially chemotherapy), viral infections, bone marrow failure, liver disease, alcohol excess, autoimmune conditions, or an enlarged spleen. 

Neutrophils are your first line of defence against bacterial infection. They make up the largest proportion of white blood cells. 

• A high neutrophil count usually suggests bacterial infection, inflammation, or physical stress on the body. 
• A low neutrophil count (neutropenia) raises your risk of infection. 

Important: The normal level of neutrophils varies between ethnic groups. Your doctor will take this into account when looking at your results. 

Lymphocytes play a key role in your immune response, particularly against viral infections. 

• A high lymphocyte count can occur with viral infections or conditions such as chronic lymphocytic leukaemia. 
• A low lymphocyte count can also be seen with certain viral infections and some other conditions. 

Monocytes help fight infection and are involved in the immune response. 

• A high monocyte count can be seen with some infections such as tuberculosis (TB), and in bone marrow disorders such as myelodysplasia. 

Eosinophils are involved in allergic responses and fighting certain parasites. 

• A high eosinophil count can occur with allergic conditions (such as asthma, hay fever, or eczema), drug allergies, some parasitic infections, vasculitis, Hodgkin’s lymphoma, and bone marrow disorders. 

Basophils are the least common type of white blood cell and are involved in inflammatory reactions. 

• A high basophil count can be seen with some infections, inflammatory conditions, and bone marrow disorders such as chronic myeloid leukaemia. 

Platelets are tiny cells that help your blood to clot when you have an injury. 

This counts the number of platelets in your blood. 

• A high platelet count can occur after bleeding, infection, inflammation, or surgery, and in bone marrow disorders. It is also seen in people whose spleen has been removed or is not working properly. 
• A low platelet count (thrombocytopenia) can be caused by immune conditions such as immune thrombocytopenic purpura (ITP) or lupus (SLE), vitamin deficiencies, alcohol excess, liver disease, some drugs (particularly chemotherapy), an enlarged spleen, bone marrow disorders, and some rare inherited conditions. 

MPV measures the average size of your platelets. Younger platelets are larger. 

• A high MPV can suggest your body is producing more platelets than usual, for example after significant blood loss or in response to low platelet numbers. 

Many FBC results that fall slightly outside the normal range do not require treatment. Your doctor may simply repeat the test after a short period to check whether the result has changed. 

If a result suggests anaemia, your doctor will look at the red cell measurements together to work out the likely cause. They may request further tests such as iron studies, vitamin B12, or folate levels. 

If the white blood cell count is significantly raised or lowered, your doctor may request additional tests to look for infection, bone marrow problems, or blood disorders. A blood film where your blood cells are looked at under a microscope is often used alongside the FBC to get more information. 

If your platelet count is very high or very low, further investigation may be needed. Your doctor will work with you to decide on the most appropriate next steps. 

Several things can affect your FBC results that are not related to illness: 

• age and sex: normal ranges are different for children, adults, and older people, and differ between males and females 
• recent blood transfusion can temporarily change your results 
• medicines: many drugs can raise or lower white blood cell counts; some can affect platelet counts 
• vitamin and mineral deficiencies, particularly iron, vitamin B12, and folate, which affect red blood cell measurements 
• alcohol: heavy drinking can affect red blood cells, white blood cells, and platelets 

Always tell your doctor about all the medicines and supplements you are taking before having this test.

Depending on your FBC results, your doctor may request: 

• Blood film: to look at the shape and appearance of your blood cells under a microscope
• Iron studies: checks your iron levels if anaemia is suspected
• Vitamin B12 and folate: checks for vitamin deficiencies that can cause anaemia
• Reticulocyte count: measures young red blood cells to see how quickly your bone marrow is producing new cells
• Bone marrow biopsy: rarely needed, but can investigate serious blood disorders