Full Blood Examination (FBE)
A full blood examination (FBE), also known as a full blood count (FBC) or complete blood count (CBC) in some countries, is one of the most commonly requested blood tests in healthcare. It provides a snapshot of the cells circulating in the blood, helping clinicians identify abnormalities that may indicate infection, anaemia, inflammation, bleeding, blood disorders, or problems affecting the bone marrow. While the FBE is a routine investigation, it should never be interpreted in isolation. Blood results are only one piece of the clinical picture and should always be considered alongside the patient's history, symptoms, observations, physical assessment and other investigations.
What You Need to Know
The FBE measures the three main types of blood cells: red blood cells, white blood cells and platelets. Together, these components provide information about oxygen transport, immune function and blood clotting. An abnormal result doesn't automatically indicate disease. Factors such as dehydration, pregnancy, recent surgery, medications, exercise and chronic medical conditions can all influence the results. Equally important is whether a result represents a sudden change or has been stable over time.
When interpreting an FBE, it's important to consider the overall pattern of results rather than focusing on a single number. Individual components often complement each other and provide far more information when interpreted together.
Key points
The FBE assesses red blood cells, white blood cells and platelets.
It can help identify infection, anaemia, bleeding, inflammation and many haematological disorders.
A result outside the reference range does not always indicate pathology.
The FBE should always be interpreted alongside the patient's clinical presentation and other investigations.
Understanding how each component relates to the others improves clinical interpretation.
Components of an FBE:
Haemoglobin (Hb)
Red Blood Cells (RBC)
Haematocrit (Hct)
Mean Cell Volume (MCV)
Mean Cell Haemoglobin (MCH)
Mean Cell Haemoglobin Concentration (MCHC)
Red Cell Distribution Width (RDW)
White Blood Cells (WCC):
Neutrophils
Lymphocytes
Monocytes
Eosinophils
Basophils
Platelets
Reticulocyte Count
Beyond the Basics
Haemoglobin (Hb)
Reference range
Male: 130–180 g/L (13.0–18.0 g/dL)
Female: 115–165 g/L (11.5–16.5 g/dL)
Haemoglobin is the iron-containing protein inside red blood cells responsible for carrying oxygen from the lungs to the body's tissues and transporting carbon dioxide back to the lungs. Because oxygen delivery depends largely on haemoglobin, significant abnormalities can have widespread physiological effects.
Decreased Hb: A low haemoglobin level indicates anaemia, although the underlying cause may vary considerably. Blood loss, iron deficiency, chronic kidney disease, nutritional deficiencies and bone marrow disorders are all common causes. As haemoglobin falls, patients may develop fatigue, pallor, shortness of breath, dizziness and tachycardia as the body attempts to compensate for reduced oxygen-carrying capacity.
Elevated Hb: An elevated haemoglobin level is less common but may occur with dehydration (because blood plasma liquid is reduced, leading to concentrated red blood cells), chronic hypoxia, smoking, chronic lung disease or conditions such as polycythaemia vera. In these situations, blood becomes more viscous, increasing the risk of thrombotic complications.
Haemoglobin should always be interpreted alongside the red blood cell count, haematocrit and red cell indices.
Red Blood Cell Count (RBC)
Reference range
Male: 4.5-6.5 × 10¹²/L
Female: 3.8-5.8 × 10¹²/L
The red blood cell (RBC) count measures the number of red blood cells in a given volume of blood. In Australia, it is reported as the number of cells per litre (×10¹²/L), while some countries report it as millions of cells per microlitre (µL). A healthy adult typically has around 5 million red blood cells in every microlitre of blood (5 × 10⁶ cells/µL). Together with haemoglobin, haematocrit and the red blood cell indices, the RBC count helps assess oxygen-carrying capacity, identify different types of anaemia and evaluate disorders affecting red blood cell production or survival.
Decreased RBC: A reduced RBC count commonly accompanies anaemia caused by bleeding, nutritional deficiencies or reduced bone marrow production.
Elevated RBC: An increased count may occur in dehydration, chronic hypoxia or disorders that stimulate excessive red blood cell production.
However, it’s important to note that the RBC count alone rarely provides a diagnosis. It is most useful when interpreted together with haemoglobin, haematocrit and the red blood cell indices, as these tests together can provide a more complete snapshot of the clinical picture.
Haematocrit (Hct)
Reference range
Male: 0.40-0.54 (40-54%)
Female: 0.36-0.47 (36-47%)
Haematocrit represents the proportion of total blood volume made up of red blood cells. Rather than counting individual cells, it shows how much of the blood consists of red cells compared with plasma. Haematocrit provides an indication of blood viscosity because it measures the proportion of blood volume made up of red blood cells. As this proportion increases, the blood becomes more viscous (thicker) and more resistant to flow. Conversely, a lower haematocrit means the blood contains fewer red blood cells and is generally less viscous.
Decreased haematocrit: A reduced haematocrit commonly accompanies anaemia caused by bleeding, nutritional deficiencies, chronic disease or reduced bone marrow production. It may also occur with overhydration due to haemodilution.
Elevated haematocrit: An increased haematocrit may occur with dehydration, chronic hypoxia, smoking, chronic lung disease or disorders that stimulate excessive red blood cell production, such as polycythaemia vera.
One thing to remember is that haematocrit is influenced by both red blood cell mass and plasma volume, unlike the RBC count. That's why dehydration and overhydration have a particularly noticeable effect on haematocrit. Because plasma volume can change independently of red cell mass, haematocrit should always be interpreted within the broader clinical picture.
Mean Cell Volume (MCV)
Reference range
80–100 fL
Mean cell volume (MCV) measures the average size, or volume, of red blood cells. It is considered quite a useful red blood cell indices because it helps classify anaemia based on whether red blood cells are smaller than normal (microcytic), normal in size (normocytic) or larger than normal (macrocytic). While MCV does not diagnose the underlying cause of anaemia, it significantly narrows the range of possible causes and guides further investigation.
Decreased MCV: A reduced MCV indicates microcytic red blood cells and is most commonly associated with iron deficiency anaemia and thalassaemia (a group of inherited genetic blood disorders that prevent the body from producing enough normal haemoglobin). Less common causes include anaemia of chronic disease and sideroblastic anaemia.
Normal MCV: A normal MCV indicates normocytic red blood cells. Normocytic anaemia may occur with acute blood loss, chronic disease, chronic kidney disease, haemolysis or bone marrow disorders.
Elevated MCV: An increased MCV indicates macrocytic red blood cells and is commonly associated with vitamin B12 deficiency, folate deficiency, liver disease, excessive alcohol intake and hypothyroidism. Certain medications and bone marrow disorders may also cause macrocytosis.
MCV should always be interpreted alongside the other red blood cell indices, particularly MCH, MCHC and RDW. For example, a low MCV with a high RDW is strongly suggestive of iron deficiency anaemia, whereas a low MCV with a normal RDW may indicate thalassaemia trait. Interpreting these indices together provides a more complete understanding of the underlying cause of anaemia.
Mean Cell Haemoglobin (MCH)
Reference range
27–33 pg
Mean cell haemoglobin (MCH) measures the average amount of haemoglobin contained within each red blood cell. Since haemoglobin is responsible for transporting oxygen throughout the body, MCH provides an indication of how much oxygen-carrying protein is present in each red blood cell. MCH is closely related to MCV because larger red blood cells generally contain more haemoglobin than smaller red blood cells.
Decreased MCH: A reduced MCH indicates that red blood cells contain less haemoglobin than normal. This is most commonly associated with iron deficiency anaemia and thalassaemia, where red blood cells often appear pale (hypochromic) due to their reduced haemoglobin content.
Elevated MCH: An increased MCH indicates that red blood cells contain more haemoglobin than normal. This commonly occurs in macrocytic anaemias, such as those caused by vitamin B12 deficiency, folate deficiency, liver disease or excessive alcohol intake. The increase usually reflects the larger size of the red blood cells rather than an increased concentration of haemoglobin.
MCH should always be interpreted alongside MCV and MCHC. While MCH indicates the total amount of haemoglobin within each red blood cell, MCHC measures the concentration of haemoglobin within the cell. Together, these indices help characterise different types of anaemia and identify abnormalities in red blood cell production.
Mean Cell Haemoglobin Concentration (MCHC)
Reference range
320–360 g/L
Mean cell haemoglobin concentration (MCHC) measures the average concentration of haemoglobin within each red blood cell. Unlike MCH, which measures the total amount of haemoglobin in each cell, MCHC indicates how densely packed the haemoglobin is within the cell. It helps determine whether red blood cells have a normal concentration of haemoglobin (normochromic) or a reduced concentration (hypochromic).
Decreased MCHC: A reduced MCHC indicates that red blood cells contain a lower concentration of haemoglobin than normal. This is most commonly associated with iron deficiency anaemia, where cells become hypochromic due to inadequate haemoglobin production. It may also occur in thalassaemia and other disorders affecting haemoglobin synthesis.
Elevated MCHC: An increased MCHC is uncommon because there is a physiological limit to how much haemoglobin can be packed into a red blood cell. Elevated values may occur in hereditary spherocytosis, where red blood cells become smaller and more spherical, increasing the haemoglobin concentration within each cell. It may also result from laboratory artefacts, such as haemolysis, cold agglutinins or lipaemia.
MCHC should always be interpreted alongside MCV and MCH. While MCV describes the size of the red blood cell and MCH measures the total amount of haemoglobin it contains, MCHC indicates how concentrated that haemoglobin is within the cell. Interpreting these indices together provides valuable information about the type and underlying cause of anaemia.
Red Cell Distribution Width (RDW)
Reference range
11.5–14.5%
Red cell distribution width (RDW) measures the variation in the size of circulating red blood cells. A normal RDW indicates that most red blood cells are a similar size, whereas an elevated RDW indicates greater variation in cell size (anisocytosis). RDW does not measure the average size of red blood cells—that is the role of MCV—but rather how much their sizes differ from one another.
Decreased RDW: A reduced RDW is uncommon and is generally not considered clinically significant. It simply indicates that red blood cells are very uniform in size and is rarely associated with disease.
Elevated RDW: An increased RDW indicates greater variation in red blood cell size and is commonly seen in iron deficiency anaemia, vitamin B12 deficiency, folate deficiency and mixed nutritional deficiencies. It may also occur following acute blood loss or blood transfusion, when older and newly produced red blood cells of different sizes circulate together.
RDW is most useful when interpreted alongside MCV. For example, a low MCV with a high RDW is strongly suggestive of iron deficiency anaemia, whereas a low MCV with a normal RDW is more consistent with thalassaemia trait. Together, these indices help narrow the differential diagnosis and guide further investigation into the underlying cause of anaemia.
White Blood Cell Count (WCC)
Reference range
4.0–11.0 × 10⁹/L
The white blood cell count (WCC) measures the total number of white blood cells circulating in the blood. White blood cells play a vital role in the body's immune system by helping detect, destroy and remove pathogens, damaged cells and other foreign substances. Changes in the WCC can provide important clues about infection, inflammation, immune disorders and diseases affecting the bone marrow.
Decreased WCC: A reduced WCC (leucopenia) may occur with viral infections, chemotherapy, bone marrow suppression, autoimmune disorders or certain medications. A low WCC may reduce the body's ability to fight infection, particularly when neutrophil levels are also reduced.
Elevated WCC: An increased WCC (leucocytosis) may occur with bacterial infections, inflammation, tissue injury, physiological stress, corticosteroid therapy, smoking or haematological malignancies such as leukaemia. The degree of elevation and the type of white blood cells involved often provide clues to the underlying cause.
The total WCC provides an overview of immune activity, but it does not identify which type of white blood cell is responsible for the change. For this reason, the differential white blood cell count, including neutrophils, lymphocytes, monocytes, eosinophils and basophils, provides greater diagnostic value and should always be interpreted alongside the total WCC.
Neutrophils
Reference range
2.0–7.5 × 10⁹/L
Neutrophils are the most abundant type of white blood cell and form the body's first line of defence against bacterial and fungal infections. They are among the first immune cells to migrate to sites of infection or tissue injury, where they destroy pathogens through phagocytosis (engulf and destroy pathogens and cellular debris) and the release of antimicrobial substances. Because of their rapid response, neutrophil levels often change early in the course of acute illness.
Decreased neutrophils: A reduced neutrophil count (neutropenia) may occur with chemotherapy, bone marrow suppression, certain viral infections, autoimmune disorders or medications. Severe neutropenia significantly increases the risk of serious bacterial and fungal infections, as the body's ability to mount an effective immune response is impaired.
Elevated neutrophils: An increased neutrophil count (neutrophilia) commonly occurs with bacterial infections, acute inflammation, trauma, surgery, tissue injury, corticosteroid therapy, physiological stress and smoking. Less commonly, neutrophilia may be associated with myeloproliferative disorders such as chronic myeloid leukaemia.
Neutrophils should always be interpreted within the context of the total white blood cell count and the patient's clinical presentation. For example, neutrophilia in a patient with fever and productive cough strongly supports a bacterial infection, whereas neutrophilia following surgery or corticosteroid administration may represent a normal physiological response rather than infection.
Lymphocytes
Reference range
1.0–4.0 × 10⁹/L
Lymphocytes are a type of white blood cell responsible for adaptive immunity, the body’s second line defence against specific pathogens. They help recognise and eliminate specific pathogens, produce antibodies or immunoglobulins (proteins that identify and destroy foreign invaders, such as bacteria, viruses and toxins), destroy infected or abnormal cells, and provide long-term immune memory following infection or vaccination. The three main types of lymphocytes are B cells, T cells and natural killer (NK) cells, each performing a distinct role in the immune response.
Decreased lymphocytes: A reduced lymphocyte count (lymphocytopenia) may occur with immunosuppression, corticosteroid therapy, chemotherapy, HIV infection, autoimmune disorders or severe acute illness. Reduced lymphocyte numbers may impair the body's ability to fight viral infections and develop an effective immune response.
Elevated lymphocytes: An increased lymphocyte count (lymphocytosis) is commonly associated with viral infections, such as infectious mononucleosis and cytomegalovirus infection. It may also occur with chronic infections, following certain bacterial infections such as pertussis, or in chronic lymphoid malignancies such as chronic lymphocytic leukaemia.
Lymphocyte results should always be interpreted alongside the total white blood cell count, the differential count and the patient's clinical presentation. For example, lymphocytosis is more suggestive of a viral infection when accompanied by compatible symptoms, whereas persistent or markedly elevated lymphocyte counts may warrant further investigation for an underlying haematological disorder.
Monocytes
Reference range
0.2–1.0 × 10⁹/L
Monocytes are the largest type of white blood cell and play an important role in the body's innate immune response. They circulate in the bloodstream for a short period before migrating into tissues, where they mature into macrophages (cells that phagocytose unwanted particles) or dendritic cells (sentinels found in tissues exposed to the environment, such as the skin and lungs). These cells remove pathogens and damaged tissue through phagocytosis, present antigens to lymphocytes, and help coordinate inflammation and tissue repair.
Decreased monocytes: A reduced monocyte count (monocytopenia) is uncommon but may occur with chemotherapy, bone marrow suppression, corticosteroid therapy, severe sepsis or aplastic anaemia. Low monocyte counts are rarely interpreted in isolation and are usually considered alongside changes in other white blood cells.
Elevated monocytes: An increased monocyte count (monocytosis) commonly occurs with chronic infections, autoimmune and inflammatory disorders, recovery following acute infection, and certain haematological malignancies such as chronic myelomonocytic leukaemia. Monocytosis may also be seen during the healing phase after tissue injury as monocytes assist with debris removal and tissue repair.
Monocyte results should always be interpreted alongside the total white blood cell count, the differential count and the patient's clinical presentation. While isolated abnormalities are uncommon, persistent monocytosis may indicate an underlying chronic inflammatory process, infection or bone marrow disorder requiring further investigation.
Eosinophils
Reference range
0.0–0.5 × 10⁹/L
Eosinophils are a type of white blood cell involved in allergic reactions, immune regulation and defence against parasitic infections. They help destroy parasites by releasing toxic proteins and contribute to inflammation by releasing chemical mediators during allergic and hypersensitivity reactions. Eosinophils also play a role in conditions such as asthma and eczema.
Decreased eosinophils: A reduced eosinophil count (eosinopenia) is usually of little clinical significance and is rarely investigated in isolation. It may occur during acute stress, severe infection, corticosteroid therapy or following the body's normal release of stress hormones.
Elevated eosinophils: An increased eosinophil count (eosinophilia) commonly occurs with allergic conditions such as asthma, allergic rhinitis and eczema, drug hypersensitivity reactions and parasitic infections. Less commonly, eosinophilia may be associated with autoimmune disorders, eosinophilic gastrointestinal diseases or certain haematological malignancies.
Eosinophil results should always be interpreted alongside the patient's history and clinical presentation. While mild eosinophilia is most often related to allergic disease, persistent or marked eosinophilia may warrant further investigation to identify an underlying parasitic, inflammatory or haematological cause.
Basophils
Reference range
0.0–0.2 × 10⁹/L
Basophils are the least abundant type of white blood cell and play a role in allergic, inflammatory and immune responses. They contain granules rich in histamine, heparin and other chemical mediators, which are released during allergic reactions and help promote inflammation, increase blood vessel permeability and recruit other immune cells to sites of injury or infection.
Decreased basophils: A reduced basophil count (basopenia) is generally not clinically significant and is rarely investigated in isolation. It may occur during acute infections, physiological stress, pregnancy, hyperthyroidism or corticosteroid therapy.
Elevated basophils: An increased basophil count (basophilia) is uncommon but may occur with chronic allergic or inflammatory conditions, chronic infections, hypothyroidism and myeloproliferative disorders such as chronic myeloid leukaemia. Basophilia is often interpreted alongside other abnormalities in the white blood cell differential.
Basophil results should always be interpreted within the context of the total white blood cell count, the differential count and the patient's clinical presentation. While mild elevations may accompany allergic or inflammatory conditions, persistent or marked basophilia may indicate an underlying haematological disorder and warrants further investigation.
Platelets (PLT)
Reference range
150–400 × 10⁹/L
Platelets (thrombocytes) are small cell fragments produced in the bone marrow from megakaryocytes. They play a vital role in haemostasis by rapidly adhering to damaged blood vessels and forming the initial platelet plug to help control bleeding. Platelets also release chemical mediators that activate the coagulation cascade and promote tissue repair following vascular injury.
Decreased platelets: A reduced platelet count (thrombocytopenia) increases the risk of bleeding and may result from reduced bone marrow production, increased destruction, excessive consumption or sequestration in an enlarged spleen (a life-threatening emergency where a large number of red blood cells become trapped inside the spleen). Common causes include chemotherapy, viral infections, immune thrombocytopenia (ITP), disseminated intravascular coagulation (DIC), thrombotic thrombocytopenic purpura (TTP), liver disease and certain medications.
Elevated platelets: An increased platelet count (thrombocytosis) may occur as a reactive response to infection, inflammation, iron deficiency, blood loss or following splenectomy. Less commonly, thrombocytosis may result from myeloproliferative disorders such as essential thrombocythaemia. Markedly elevated platelet counts may increase the risk of thrombotic complications, although platelet function and the underlying cause should also be considered.
Platelet results should always be interpreted alongside the patient's clinical presentation, bleeding or thrombotic history and other coagulation studies where appropriate. A normal platelet count does not necessarily indicate normal platelet function, as disorders affecting platelet function can still result in clinically significant bleeding despite a normal platelet count.
In Practice
Understanding what each component of the FBE measures is only the first step. In clinical practice, blood results are interpreted alongside the patient's history, symptoms, observations and other investigations. The following scenarios demonstrate how changes in the FBE contribute to clinical reasoning and help guide further assessment and management.
Scenario 1: The patient with bacterial sepsis
A 74-year-old man presents to the emergency department with fever, rigors, confusion and hypotension. He has a productive cough and oxygen saturations of 89% on room air.
FBE
Hb: 142 g/L
WCC: 19.8 × 10⁹/L
Neutrophils: 17.2 × 10⁹/L
Lymphocytes: 0.7 × 10⁹/L
Platelets: 280 × 10⁹/L
Interpretation
The markedly elevated WCC and neutrophil count are consistent with an acute bacterial infection. Lymphocyte counts commonly decrease during acute physiological stress and severe bacterial illness. In this patient, the FBE supports the clinical suspicion of sepsis but should always be interpreted alongside observations, lactate, blood cultures and other investigations.
Scenario 2: Iron deficiency anaemia following chronic gastrointestinal bleeding
A 68-year-old woman reports increasing fatigue, shortness of breath on exertion and dizziness. She has noticed dark stools over several weeks.
FBE
Hb: 84 g/L
RBC: 3.3 × 10¹²/L
Haematocrit: 0.28
MCV: 71 fL
MCH: 22 pg
MCHC: 305 g/L
RDW: 18.4%
WCC: 6.8 × 10⁹/L
Platelets: 510 × 10⁹/L
Interpretation
This FBE demonstrates a microcytic, hypochromic anaemia with an elevated RDW, strongly suggesting iron deficiency. The elevated platelet count is a common reactive finding in iron deficiency anaemia. Given the patient's history of melaena, chronic gastrointestinal bleeding should be investigated as the underlying cause.
Scenario 3: Pancytopenia following chemotherapy
A 56-year-old woman is receiving chemotherapy for breast cancer. She presents with fever, mouth ulcers and increasing fatigue.
FBE
Hb: 86 g/L
RBC: 2.8 × 10¹²/L
WCC: 0.9 × 10⁹/L
Neutrophils: 0.2 × 10⁹/L
Platelets: 42 × 10⁹/L
Interpretation
This patient has pancytopenia, with reductions in red blood cells, white blood cells and platelets. The profound neutropenia places her at high risk of life-threatening infection, while thrombocytopenia increases her risk of bleeding. This is an oncological emergency requiring urgent assessment and management for possible febrile neutropenia.
Scenario 4: Vitamin B12 deficiency
A 72-year-old vegetarian presents with fatigue, numbness in both feet and difficulty maintaining balance.
FBE
Hb: 96 g/L
RBC: 3.1 × 10¹²/L
Haematocrit: 0.31
MCV: 112 fL
MCH: 36 pg
MCHC: 340 g/L
RDW: 17.2%
Interpretation
The elevated MCV indicates macrocytic anaemia. Combined with the patient's neurological symptoms, vitamin B12 deficiency should be strongly suspected. Further investigations, including vitamin B12 and folate levels, would help confirm the diagnosis.
Scenario 5: Polycythaemia secondary to chronic hypoxia
A 67-year-old man with severe COPD attends his routine respiratory clinic review. He has a long smoking history and chronic hypoxaemia.
FBE
Hb: 187 g/L
RBC: 6.4 × 10¹²/L
Haematocrit: 0.57
MCV: 89 fL
WCC: 8.4 × 10⁹/L
Platelets: 295 × 10⁹/L
Interpretation
The elevated haemoglobin, RBC count and haematocrit indicate erythrocytosis. In the context of chronic hypoxaemia from COPD, this is most consistent with secondary polycythaemia. Increased red blood cell production improves oxygen-carrying capacity but also increases blood viscosity, potentially increasing the risk of thrombotic complications.