11/13/24, 7\:32 PM Guide | FBC interpretation

FBC interpretation

Table of contents

Introduction

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 di
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
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\:
Haemoglobin (Hb)
Haematocrit (Hct)
Mean corpuscular volume (MCV)
Red cell distribution width (RDW)
Red cell count (RCC)
Reticulocyte count
Mean corpuscular haemoglobin (MCH)
Mean corpuscular haemoglobin concentrate (MCHC)

White cell tests

White cell tests include\:
White blood cell count (WCC)
White blood cell di

Platelet tests

Platelet tests include\:
Platelet count
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
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results tend to be used in more speci

Reference ranges

We have included key reference ranges below to assist you when interpreting a full blood count.
Haemoglobin (Hb)\:
♂ 130 – 180 g/ L
♀ 115 – 165 g/ L
White cell count (WCC)\:
9
Total\: 3.6 – 11.0 x 10 /L
9
Neutrophils\: 1.8 – 7.5 x 10 /L
9
Lymphocytes\: 1.0 – 4.0 x 10 /L
9
Monocytes\: 0.2 – 0.8 x 10 /L
9
Eosinophils\: 0.1 – 0.4 x 10 /L
9
Basophils\: 0.02 – 0.10 x 10 /L
9
Platelet count\: 140 – 400 x10 /L
Red cell count (RCC)\:
♂ 4.5 – 6.5 x 10 12
/L
♀ 3.8 – 5.8 x 10 12
/L
Haematocrit\:
♂ 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-specireference 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.

Haemoglobin (Hb)

Anaemia
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
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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 di
In general, when you identify anaemia it is sensible to check haematinics (e.g. ferritin, B12/folate) as de
and easy to treat.
Polycythaemia
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.
Primary polycythaemia is typically caused by myeloproliferative neoplasms (e.g. polycythaemia rubra vera).
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\:
Chronic obstructive pulmonary disease
Smoking
Obstructive sleep apnoea
Cyanotic heart disease
Lung
Exogenous steroids
Excess alcohol intake (can also cause low blood cell counts)
Certain malignancies (renal cell carcinoma, cerebellar haemangioma, Wilm’s tumour)
EPO abuse (e.g. in athletics)
Endogenous steroids (Conn's syndrome, Cushing's syndrome)
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
intake or states of excess
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 di
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 de
Microcytic anaemia
Causes of microcytic anaemia include\:
Iron deiron de)
Haemoglobinopathies (e.g. thalassemia syndrome/trait)
Anaemia of chronic disease/in
Lead poisoning (rare)
Sideroblastic anaemia (rare)
Hookworm infection (a common cause of microcytic anaemia in low/middle-income countries)
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Hint\: An acronym to remember the causes of microcytic anaemia is TAILS\: Thalassemia/haemoglobinopathies, Anaemia
of chronic disease/in
Normocytic anaemia
Causes of normocytic anaemia include\:
Anaemia of chronic disease/in
Acute blood loss
Increase plasma volume (e.g. pregnancy,
Mixed aetiology anaemias
Haemoglobinopathies (e.g. thalassaemias)
Aplastic anaemia
Haemolysis
Hypersplenism (leads to increased destruction of red blood cells)
Macrocytic anaemia
Causes of macrocytic anaemia include\:
B12/folate de
Toxins (e.g. alcohol, chemotherapy)
Liver disease
Reticulocytosis (reticulocytes are immature red blood cells)
Pregnancy
Myeloma
Myelodysplastic syndrome
Hypothyroidism

Haematocrit (Hct)

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 a
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 speci
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 con
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 de
(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 de

Reticulocyte count

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.
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A raised reticulocyte count in the context of anaemia implies that the bone marrow is e
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 de
aplastic anaemia, bone marrow in
A raised reticulocyte count in the absence of anaemia may indicate that the body is e
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

Leukocytosis

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.
Common causes of acute leukocytosis\:
Reactive\: infection, in
Steroids\: stress response (i.e. endogenous steroids) or medication (i.e. exogenous steroids)
Haematological\: acute leukaemias
Common causes of chronic leukocytosis\:
Reactive\: chronic infection, smoking
Haematological\: leukaemia, certain subtypes of lymphoma
Hyposplenism\: typically mild
Pregnancy
Neutrophils and lymphocytes make up the majority of the white cells in the blood, so these two cell types are usually the
cause of a raised total white cell count.
A comprehensive assessment is always required to help narrow the di
for further investigations.

Leukopenia

A low white cell count is known as leukopenia and can also be caused by a wide range of pathological processes.
Important factors in determining the cause of leukopenia include the rate of the white cell count fall and the type of white
cell(s) which are low.
Neutrophils and lymphocytes account for the majority of white cells in the blood, so these types of white cells are typically
reduced in the context of leukopenia.
A comprehensive assessment is always required to help narrow the di
further investigations.
Common causes of leukopenia include\:
Infection\: can be seen as a transient phenomenon in viral illness or as a result of consumption in sepsis
Medications\: antibiotics, immunosuppressants, anti-epileptics, cytotoxic agents (e.g. chemotherapy)
B12/folate de
Autoimmune disease
Iron de
HIV (any cytopenia could be due to HIV)
Racial variation\: middle eastern and black patients can have lower baseline neutrophil counts which are not pathological
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Bone marrow failure\: often this will be seen alongside low platelets and low haemoglobin

White cell di

Neutrophils
Neutrophils usually account for 40-70% of the total white blood cells and are key in the acute phase of infection (particularly
bacterial infections).
Neutrophilia
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.
Neutropenia
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
Lymphocytes make up between 18-42% of white cells found in circulating blood. There are various subtypes with slightly
diimmune system, including
Lymphocytosis
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\:
Viral infection
Smoking
Hyposplenism/post-splenectomy
Malignancy\: leukaemia and certain types of lymphoma
Pertussis\: rates are increasing in the U.K. with decreased vaccination rates
Lymphopenia
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\:
Infection
Older age (rarely clinically signi
Alcohol excess
HIV
Autoimmune disease
Bone marrow disease
Medications\: cytotoxic agents, immunosuppressants
Renal failure
Congenital immunode
Monocytes
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.
Monocytosis
A high monocyte count (monocytosis) is usually an acute
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Common causes of monocytosis include\:
Bacterial infection
Autoimmune disease
Steroids
Monocytopenia
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\:
Acute infection
Steroids
Bone marrow failure
Cytotoxic agents (e.g. chemotherapy)
Hairy cell leukaemia
Eosinophils
Eosinophils account for between 2-3% of the total white cell count. They migrate to sites of in
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.
Eosinophilia
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.
Causes of eosinophilia include\:
Allergies/atopy
Parasitic infection
Autoimmune disease (e.g. vasculitis)
Medications (e.g. antibiotics, anti-epileptics, allopurinol)
Gastrointestinal disease (e.g. eosinophilic oesophagitis)
Respiratory disease (e.g. asthma)
Malignancy (any solid organ or haematological)
Basophils
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 inimmune response, as well as heparin
which prevents blood from clotting improving the blood supply to the in
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 signi
Basophilia
Transient, isolated basophilia is rarely clinically signi
basophilia is present alongside elevated neutrophil and eosinophil counts or if the elevation is signi
range, this should raise the suspicion of a myeloproliferative disorder and prompt discussion with a haematologist.
Causes of basophilia include\:
Allergic reactions/atopy
Iron de
Chronic in
Hypothyroidism
Infection
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Haematological malignancies (myeloproliferative disorders)
Blasts
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
Causes of an elevated blast count include\:
Acute leukaemia
Myeloproliferative disorders
Reactive (severe infection or treatment with G-CSF)
Cytotoxic agents (chemotherapy)

Platelet tests

Platelet count

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.
Causes of acute thrombocytopenia\:
Consumption (e.g. infection, bleeding)
Acute viral infection
Medications (e.g. antibiotics, anti-epileptics, cytotoxic agents)
Disseminated intravascular coagulation/microangiopathic haemolytic anaemia (e.g. TTP, HUS)
Heparin-induced thrombocytopenia (HIT)
Immune thrombocytopenic purpura (ITP)
Pregnancy\: pre-eclampsia/HELLP syndrome
Causes of chronic thrombocytopenia\:
Hypersplenism
Cirrhosis
Alcohol excess
Medications (e.g. anti-epileptics, cytotoxic agents)
ITP
Autoimmune disease
B12/folate de
Iron de
HIV
Hepatitis B/C
Haematological disease
Bone marrow failure
Thrombocytosis
A raised platelet count is known as thrombocytosis and has lots of potential causes.
Causes of thrombocytosis include\:
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Reactive\: in
Myeloproliferative disorders\: typically essential thrombocythaemia although any myeloproliferative disorder can elevate
platelet counts
Iron de
Hypospenlism/post-splenectomy
Underlying malignancy\: likely secondary to underlying in
Hint\: When an elevated platelet count is reactive, there is no increased clotting risk and the count tends to normalise
after treatment of the underlying cause.

Reviewers

Dr Hannah Bielby
Haematology Registrar
Dr Callum Wright
Haematology Registrar
Source\: geekymedics.com
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