What Is a Full Blood Count?
A full blood count (FBC), also known as a complete blood count (CBC), is a laboratory test that measures various characteristics of blood cells.
The main parameters assessed relate to three types of blood cells:
- Red blood cells: Responsible for transporting oxygen around the body.
- White blood cells: Play a role in immune defence.
- Platelets: Help control bleeding and form blood clots when blood vessels are damaged.
A full blood count is a useful initial test when investigating a wide range of medical conditions. However, it cannot determine the underlying cause of an abnormality on its own. [1]
What Do Red Blood Cell (RBC) Results Show?
Red blood cells contain haemoglobin, the protein responsible for binding and transporting oxygen to the body’s tissues.
A full blood count measures several parameters related to these cells.
Haemoglobin (Hb or HGB)
Haemoglobin measures the concentration of haemoglobin in the blood.
A low haemoglobin level is a key finding in the diagnosis of anaemia. However, anaemia can have several causes, including iron deficiency, vitamin B12 deficiency, chronic diseases and other conditions.
Haematocrit (HCT)
Haematocrit represents the percentage of total blood volume occupied by red blood cells.
It is influenced by both red blood cell volume and plasma volume. For example, dehydration can cause a relative increase in haematocrit.
Red Blood Cell Count (RBC)
This measures the number of red blood cells in a given volume of blood.
The result is interpreted alongside haemoglobin, haematocrit and other red blood cell indices. [1]
What Do MCV, MCH and RDW Mean?
These indices help assess the characteristics of red blood cells.
MCV (Mean Corpuscular Volume)
MCV measures the average size of red blood cells.
- Low MCV: May be seen in iron deficiency or thalassaemia.
- High MCV: May be associated with vitamin B12 or folate deficiency, as well as other conditions.
MCH (Mean Corpuscular Haemoglobin)
MCH measures the average amount of haemoglobin in each red blood cell.
It may be reduced in iron deficiency, although this finding is not specific to the condition.
RDW (Red Cell Distribution Width)
RDW describes the variation in red blood cell size.
An elevated RDW indicates greater variation in cell size, but does not identify the underlying cause on its own.
These indices are useful when investigating anaemia. However, neither a low MCV nor a low MCH is sufficient on its own to confirm iron deficiency. [1], [2]
Further testing, including ferritin and other markers of iron status, may be needed to investigate possible iron deficiency.
Adequate dietary iron intake is also important.
What Do White Blood Cell (WBC) Results Show?
White blood cells help protect the body against infections and perform various other immune functions.
High white blood cell counts (leukocytosis) may occur during infections, inflammatory conditions, or in response to certain medications and other factors.
Low white blood cell counts (leukopenia) may occur with certain viral infections, as a result of medication use or due to disorders affecting blood cell production.
However, the total white blood cell count does not always provide enough information.
When a white blood cell differential is requested, the laboratory also measures the different types of white blood cells, including neutrophils, lymphocytes, monocytes, eosinophils and basophils.
For example, neutrophils play an important role in the body’s defence against bacterial infections. [1]
In some cases, healthcare professionals may also assess additional markers, such as C-reactive protein (CRP).
What Do Platelet (PLT) Results Show?
Platelets are small cell fragments that play an essential role in haemostasis, the process that helps stop bleeding.
Low platelet counts (thrombocytopenia) may be associated with an increased risk of bleeding, particularly when platelet levels are significantly reduced.
High platelet counts (thrombocytosis) may occur in response to inflammation, iron deficiency or other conditions. Less commonly, they may be associated with primary haematological disorders.
It is important to recognise that an unexpected result may need to be confirmed.
For example, platelet clumping within a blood sample can sometimes produce a falsely low platelet count. [1]
How Should Normal Blood Count Values Be Interpreted?
There is no single reference range that applies to everyone.
Reference ranges depend on several factors, including age, sex, laboratory methods, and the population used to establish them.
Recommendations from the International Council for Standardization in Haematology (ICSH) emphasise the importance of using appropriate laboratory reference ranges. [3]
A study published in Nature in 2025 also found that several blood cell parameters remain relatively stable within individuals over long periods of time in healthy adults. [4]
This highlights the value of comparing a patient’s current blood test results with their previous results, when available.