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This guide provides a structured approach to the interpretation of a full blood count (a.k.a. FBC, complete blood count, CBC) which you should be able to apply in most circumstances.
Note: This guide is only relevant to non-pregnant adults as normal ranges differ significantly in pregnant patients and in the paediatric population.
What tests are included in a FBC?
The full blood count gives insight into the cellular components of blood including white blood cells, red blood cells, and platelets providing numerical values relating to these cells (e.g. amount of them, size, contents). These results provide vital clues to the presence of underlying pathology. The long list of acronyms and numbers can seem daunting at first, however, by following a structured approach you can make sense of them all!
A standard FBC can be broken down into the following red cell, white cell and platelet tests.
Red cell tests
Red cell tests include:
Mean corpuscular volume (MCV)
Red cell distribution width (RDW)
Red cell count (RCC)
Mean corpuscular haemoglobin (MCH)
Mean corpuscular haemoglobin concentrate (MCHC)
White cell tests
White cell tests include:
White blood cell count (WCC)
White blood cell differential
Platelet tests include:
Mean platelet volume (MPV)
Platelet distribution width (PDW)
Tip: Those tests shown in bold are the most important to understand and those that this article will focus on. However, some laboratories will provide results for all the above and more as part of a standard full blood count. These additional results tend to be used in more specific situations by specialists.
We have included key reference ranges below to assist you when interpreting a full blood count.
♂ 130 – 180 g/L
♀ 115 – 165 g/L
White cell count (WCC):
Total: 3.6 – 11.0 x 109/L
Neutrophils: 1.8 – 7.5 x 109/L
Lymphocytes: 1.0 – 4.0 x 109/L
Monocytes: 0.2 – 0.8 x 109/L
Eosinophils: 0.1 – 0.4 x 109/L
Basophils: 0.02 – 0.10 x 109/L
Platelet count: 140 – 400 x109/L
Red cell count (RCC):
♂ 4.5 – 6.5 x 109/L
♀ 3.8 – 5.8 x 109/L
♂ 0.40 – 0.54 L/L
♀ 0.37 – 0.47 L/L
Mean cell volume (MCV): 80 – 100 fL
Mean corpuscular haemoglobin(MCH): 27 – 32 pg/cell
Reticulocyte count: 0.2 – 2%
Note: Reference ranges are lab-specific, therefore always refer to your local reference ranges when interpreting a full blood count.
Red cell tests
Red cell tests can quantify the amount of haemoglobin/red cells present in a sample:
An abnormally low number of haemoglobin/red cells is known as anaemia
An abnormally high number of haemoglobin/red cells is known as polycythaemia.
Red cell tests can also provide important insights into the underlying cause(s) of anaemia or polycythaemia by looking at the average size of the cells and how much haemoglobin is contained within them.
Anaemia refers to a decrease in the total amount of haemoglobin in the blood.
There are a wide range of causes of anaemia, which can be sub-categorised based on the average red cell size (mean cell volume/MCV):
Microcytic anaemia: low haemoglobin associated with a reduced MCV
Macrocytic anaemia: low haemoglobin associated with an increased MCV
Normocytic anaemia: low haemoglobin associated with a normal MCV
When you identify a low haemoglobin, you should look to the MCV to see which sub-type of anaemia is present as this information, alongside a good clinical assessment, can help narrow the differential diagnosis.
In general, when you identify anaemia it is sensible to check haematinics (e.g. ferritin, B12/folate) as deficiencies are common and easy to treat.
Polycythaemia is an abnormally high haemoglobin concentration, this is usually seen alongside an elevated haematocrit (the percentage of blood that is made up by red cells). Polycythemia can be due to an increase in the number of red cells, termed absolute polycythaemia, or it can be caused by a decrease in the amount of blood plasma (the liquid component of blood), which is termed relative polycythaemia. Absolute polycythemia can be caused by an issue in the bone marrow leading to a proliferation of red cell precursors (primary polycythaemia) or conditions that increase the amount of erythropoietin (EPO) circulating in the blood (secondary polycythaemia). EPO is a hormone produced in the kidneys which stimulates the bone marrow to make more red blood cells. As a result, when there is a state of EPO excess, this leads to polycythemia.
Secondary polycythaemia has a wide range of possible causes, some of these create a chronic state of hypoxia while some are due to ectopic EPO production or stimulation. Some examples of causes of secondary polycythaemia include:
Relative polycythaemia involves a reduction in the volume of blood plasma causing red blood cells to become more concentrated while the actual number of them does not change (like putting less water into cordial would make a more concentrated drink, less plasma in the blood sample will concentrate the red blood cells). This can be caused by low fluid intake or states of excess fluid loss, such as significant burns.
Tip: Most polycythaemia is secondary, with the most common causes being smoking and excess alcohol intake.
Mean corpuscular volume (MCV)
Mean corpuscular volume (MCV) is a measure of the average size of the red cells present in the blood sample. This test is particularly useful in the context of anaemia, where it can be used to aid the differential diagnosis of potential underlying causes and guide which investigations to perform next.
The following list of underlying causes are ‘textbook’ but there is often some overlap between them. For instance, don’t assume a patient with a macrocytosis cannot also be iron deficient.
Anaemia of chronic disease/inflammation (can be microcytic, but often normocytic)
Lead poisoning (rare)
Sideroblastic anaemia (rare)
Hookworm infection (a common cause of microcytic anaemia in low/middle-income countries)
Hint: An acronym to remember the causes of microcytic anaemia is TAILS: Thalassemia/haemoglobinopathies, Anaemia of chronic disease/inflammation, Iron deficiency anaemia, Lead poisoning, Sideroblastic anaemia
Hypersplenism (leads to increased destruction of red blood cells)
Causes of macrocytic anaemia include:
Toxins (e.g. alcohol, chemotherapy)
Reticulocytosis (reticulocytes are immature red blood cells)
Haematocrit (Hct) is the percentage of the blood sample that is made up of red cells and tends to closely follow the trend of the haemoglobin level and red cell count. Haematocrit can be affected by either the number of red blood cells or the volume of blood plasma.
A raised haematocrit can result in increased blood viscosity due to there being a high number of red cells relative to blood plasma making blood ‘sticky’. If the haematocrit continues to rise, hyperviscosity syndrome can develop, which is associated with the development of thrombi and symptoms such as headaches, blurred vision and chest pain. To prevent hyperviscosity some patients require venesection to reduce their haematocrit to safe levels with specific target levels varying depending on the underlying cause of the polycythemia.
Red cell count
Red cell count is the number of red cells present per unit volume of blood. This test can be used in combination with the haemoglobin level and the haematocrit to confirm the presence of anaemia or polycythaemia. The causes of a high or low red cell count are the same as those for haemoglobin and haematocrit.
Red cell distribution width (RDW)
Red cell distribution width (RDW) is useful to take a more in-depth look at the MCV (average of the red cell size). The RDW is a range from the largest red cell present to the smallest red cell present. This is useful information in the context of mixed anaemia (i.e. anaemia involving the presence of both macrocytic red cells and microcytic red cells). Mixed anaemia may develop in conditions where absorption from the gut is impaired, such as coeliac disease, leading to deficiencies in both iron (which typically causes microcytic anaemia) and B12/folate (which typically causes macrocytic anaemia). In these situations, the MCV can be misleadingly normal, however, this large variation in red cell size will be evident with an elevated RDW. The presence of red cells of varying sizes is known as anisocytosis and can be associated with iron deficiency anaemia as well.
A reticulocyte is an immature red cell that is normally released from the bone marrow into the peripheral blood, fully maturing into a red blood cell within 24 hours. Therefore, the reticulocyte count can be used to assess the response of the bone marrow to anaemia.
A raised reticulocyte count in the context of anaemia implies that the bone marrow is effectively functioning to produce lots of new red blood cells to correct the anaemia. This would therefore suggest red blood cells are being destroyed in the peripheral circulation (e.g. haemolysis, bleeding) rather than there being an issue with the production of red blood cells in the bone marrow itself.
Whereas, a low reticulocyte count in the context of anaemia implies a problem with the bone marrow not being able to make enough cells. This could be due to nutritional deficiencies (e.g. B12/folate or iron) or a primary bone marrow disorder (e.g. aplastic anaemia, bone marrow infiltration from solid organ malignancies).
A raised reticulocyte count in the absence of anaemia may indicate that the body is effectively compensating for blood loss or haemolysis (i.e. the increased production is managing to replenish the number of cells being lost in the peripheral circulation). Alternatively, a raised reticulocyte count in the absence of anaemia may be due to the body adapting to increased oxygen demands.
White cell tests
A high white cell is known as leukocytosis and can be caused by a wide range of pathological processes. Important factors in determining the cause of leukocytosis include the rate of the white cell count rise and which type of white cell(s) are raised.
Racial variation: middle eastern and black patients can have lower baseline neutrophil counts which are not pathological
Bone marrow failure: often this will be seen alongside low platelets and low haemoglobin
White cell differential
Neutrophils usually account for 40-70% of the total white blood cells and are key in the acute phase of infection (particularly bacterial infections).
As most of the white cells present in the blood are neutrophils, the causes of a high neutrophil count are the same as those that cause a high total white cell count.
As most of the white cells present in the blood are neutrophils, the causes of a low neutrophil count are the same as those that cause a low total white cell count.
Hint: When neutrophil counts are low (<1.5) there is a higher infection risk. For this reason, if a neutropenic patient is febrile it is a medical emergency that requires prompt treatment with broad-spectrum intravenous antibiotics. This is known as neutropenic sepsis or febrile neutropenia.
Lymphocytes make up between 18-42% of white cells found in circulating blood. There are various subtypes with slightly different functions which play key roles in many aspects of the immune system, including fighting viral infections.
A raised lymphocyte count is not uncommon and is typically due to a benign cause (e.g. viral infection). However, in some cases, a raised lymphocyte count may be caused by an underlying haematological malignancy.
Causes of lymphocytosis include:
Malignancy: leukaemia and certain types of lymphoma
Pertussis: rates are increasing in the U.K. with decreased vaccination rates
Lymphopenia is usually due to a benign transient cause (e.g. infection) and rarely causes any long term issues. However, it can be also indicative of more serious underlying pathology.
Causes of lymphopenia include:
Older age (rarely clinically significant in this context)
Monocytes make up about 4% of the total white cell count. They also play an important role in the immune system, becoming macrophages or dendritic cells in the periphery and taking part in phagocytosis, antigen presentation and cytokine production.
A high monocyte count (monocytosis) is usually an acute finding in response to inflammation or infection.
Commoncauses of monocytosis include:
A low monocyte count (monocytopenia) is not usually clinically relevant unless it is very low (i.e. near 0).
Causes of a very low monocyte count include:
Bone marrow failure
Cytotoxic agents (e.g. chemotherapy)
Hairy cell leukaemia
Eosinophils account for between 2-3% of the total white cell count. They migrate to sites of inflammation and are key in the immune response to helminth infections. Once activated, eosinophils release reactive oxygen species and enzymes causing local tissue damage. Eosinophils are implicated alongside basophils and mast cells in the underlying pathophysiology of asthma. They also have various non-immunological functions which are beyond the scope of this article.
Due to eosinophils making up such a small percentage of white cells, a reduced number of eosinophils is not usually clinically relevant.
There is a wide range of possible causes of a raised eosinophil count (eosinophilia). As a result, a comprehensive clinical assessment is required to guide any potential further investigations.
Basophils are the least common type of white blood cell making up just 0.5-1% of the total white cell count. They have granules containing histamine and serotonin which promote inflammation as part of the immune response, as well as heparin which prevents blood from clotting improving the blood supply to the inflamed tissue. Like eosinophils, basophils play key roles in parasitic infections and allergies. Basophils bind IgE to a receptor on their cell surface acting as part of the selective response to environmental substances. As basophils make up such a low percentage of total white cells in healthy individuals, there is usually little clinical significance in a low basophil count.
Transient, isolated basophilia is rarely clinically significant as it can be caused by many benign processes. However, when basophilia is present alongside elevated neutrophil and eosinophil counts or if the elevation is significantly above the normal range, this should raise the suspicion of a myeloproliferative disorder and prompt discussion with a haematologist.
Blasts are immature cells that are usually only found in the bone marrow where they complete their maturation before being released into the circulation. The presence of high numbers of blasts in the circulating blood is abnormal and could be caused by leukaemia, hence this should prompt an urgent haematology assessment including a blood film and a bone marrow biopsy.
Causes of an elevated blast count include:
Reactive (severe infection or treatment with G-CSF)
Cytotoxic agents (chemotherapy)
Platelets (a.k.a. thrombocytes) are disc-shaped cell fragments that react to blood vessel injury by clumping to initiate the formation of a blood clot.
The platelet count is the number of platelets present per unit volume of blood. This can be high (thrombocytosis) or low (thrombocytopenia).
Low platelet count
A reduced platelet count (thrombocytopenia) increases the risk of bleeding due to a reduced ability to form blood clots. Clinical features of thrombocytopenia include mucosal bleeding (e.g. epistaxis, bleeding gums) and spontaneous bruising.
The causes of a low platelet count can be grouped into acute and chronic.