The full blood count (FBC), sometimes referred to as a full blood examination or complete blood count, is one of the most commonly performed blood tests, as it can tell us so much about the status of our health. It is important for diagnosing conditions in which the number of blood cells is abnormally high or abnormally low, or the cells themselves are abnormal.
A full blood count measures the status of a number of different features of the blood, including:
The following provides an explanation of the various components that are measured, and helps to demystify some of the jargon you may hear in relation to this blood test.
Haemoglobin is an iron-containing compound found in the red blood cells, which transports oxygen around the body. Measuring the concentration of haemoglobin in the blood can help diagnose anaemia, a condition caused by a deficiency of haemoglobin. Anaemia can arise due to:
This measurement may also detect abnormally high concentrations of haemoglobin. This may occur in people with chronic lung disease, as an adaptation to high altitudes, or because of an abnormal increase in red cell production by the bone marrow (polycythaemia vera).
Red cell count is an estimation of the number of red blood cells per litre of blood.
Abnormally low numbers of red blood cells may indicate anaemia as a result of blood loss, bone marrow failure, malnutrition such as iron deficiency, over-hydration, or mechanical damage to red blood cells.
Abnormally high numbers of red blood cells may indicate congenital heart disease, some lung diseases, dehydration, kidney disease or polycythaemia vera.
Haematocrit is a measure of the percentage of red blood cells to the total blood volume.
A low haematocrit may indicate anaemia, blood loss, bone marrow failure, leukaemia, multiple myeloma, nutritional deficiency, over-hydration or rheumatoid arthritis.
A high haematocrit may indicate dehydration (for example, due to burns or diarrhoea), eclampsia (a serious condition that can occur during pregnancy) or polycythaemia vera.
Mean cell volume is an estimate of the volume of red blood cells. It is useful for determining the type of anaemia a person might have.
A low MCV may indicate iron deficiency, chronic disease, pregnancy, a haemoglobin disorder such as thalassaemia, anaemia due to blood cell destruction or bone marrow disorders.
A high MCV may indicate anaemia due to nutritional deficiencies, bone marrow abnormalities, liver disease, alcoholism, chronic lung disease, or therapy with certain medications.
These measures, also known as mean corpuscular haemoglobin and mean corpuscular haemoglobin concentration, are further guides to the investigation of anaemia.
The MCH is the haemoglobin content of the average red cell. The MCHC is the average haemoglobin concentration in a given volume of packed red cells.
The MCH may be low in types of anaemia where the red blood cells are abnormally small, or high in other types of anaemia where the red blood cells are enlarged (for example, as a result of folic acid or vitamin B12 deficiency).
The MCHC is low in iron deficiency, blood loss, pregnancy and anaemias caused by chronic disease.
White cell count estimates the total number of white blood cells per litre of blood.
An abnormal high or low white cell count can indicate many possible medical conditions and a leucocyte differential count, which provides numbers of the different types of white cells, is usually needed to help make any diagnosis.
Abnormally low numbers of white blood cells may indicate liver or spleen disorders, bone marrow disorders, or exposure to radiation or toxic substances. A number of viral infections can cause a temporary reduction in the white cell count.
Abnormally high levels of white blood cells may indicate infection, tissue damage, leukaemia, or inflammatory diseases.
Leucocyte differential count provides an estimate of the numbers of the 5 main types of white blood cells. These are: neutrophils; monocytes; lymphocytes; eosinophils; and basophils.
Each of the 5 types has a specific role in the body.
Neutrophils and monocytes protect the body against bacteria and eat up small particles of foreign matter.
Lymphocytes are involved in the immune process, producing antibodies against foreign organisms, protecting against viruses and fighting cancer.
Eosinophils kill parasites and are involved in allergic responses. High numbers of eosinophils may be associated with worm infections or exposure to substances that cause allergic reactions.
Basophils also take part in allergic responses and increased basophil production may be associated with bone marrow disorders or viral infection.
The normal ranges for the number of the different types of white cells in adults are:
Platelet count is an estimation of the number of platelets per litre of blood. Abnormally low numbers of platelets is known as thrombocytopenia, while an abnormally high level of platelets is known as thrombocytosis.
Platelet counts are often used to monitor medications that can have toxic effects on bone marrow, or conditions such as thrombocytopenia. They may also be used to help diagnose problems associated with abnormal bleeding or bruising.
The ranges for what is considered normal given here are typical figures. However, different laboratories may use slightly different ranges, depending on how they perform the measurements. Ask your doctor if you are unsure about your results.
Last Reviewed: 30 August 2009
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