Full Blood Count
At a Glance
Why Get Tested?
To determine general health status and to screen for a variety of disorders, such as anaemia and infection, inflammation nutritional status and exposure to toxic substances
When to Get Tested?
As part of a routine medical examination or as determined by your doctor
Sample Required?
A blood sample taken from a vein in the arm or a finger-prick or heel-prick (newborns)
Test Preparation Needed?
None
The Test Sample
What is being tested?
The full blood count (FBC) is one of the most commonly ordered tests and provides important information about the kinds and numbers of cells in the blood: red blood cells, white blood cells and platelets. Abnormalities in any of these types of cells can indicate the presence of important medical disorders.
Blood is composed of a variety of living cells that circulate through the heart, arteries and veins carrying nourishment, hormones, vitamins, antibodies, heat and oxygen to the body's tissues. Blood contains three main components — red blood cells, white blood cells, and platelets — suspended in fluid, called plasma. Red blood cells contain haemoglobin, a protein that carries oxygen to all the tissues of the body. Among other functions, white blood cells are responsible for protecting the body from invasion by foreign substances such as bacteria, fungi and viruses. White blood cells also control the immune process. Platelets help the blood clotting process by plugging holes in broken blood vessels.
How is the sample collected for testing?
The FBC is performed on a blood sample taken by a needle placed in a vein in the arm or by a finger-prick (for children and adults) or heel-prick (for infants).
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
The Test
How is it used?
The FBC is used as a broad screening test to check for such disorders as anaemia, infection, and many other diseases. It is actually a panel of tests that examines different parts of the blood and includes the following:
- White blood cell (WBC) count is a count of the actual number of white blood cells per volume of blood. Both increases and decreases can be significant.
- White blood cell differential: looks at the types of white blood cells present. There are five different types of white blood cells, each with its own function in protecting us from infection. The differential classifies a person's white blood cells into each type: neutrophils (also known as PMNs or neuts), lymphocytes, monocytes, eosinophils, and basophils.
- Red blood cell (RBC) count: is a count of the actual number of red blood cells per volume of blood. Both increases and decreases can point to abnormal conditions.
- Haemoglobin measures the amount of oxygen-carrying protein in the blood.
- Haematocrit measures the amount of space red blood cells take up in the blood. It is reported as a percentage (0 to 100) or a proportion (0 to 1).
- The platelet count is the number of platelets in a given volume of blood. Both increases and decreases can point to bleeding or clotting disorders.
- Mean platelet volume (MPV) is a machine-calculated measurement of the average size of your platelets. New platelets are larger, and an increased MPV occurs when increased numbers of platelets are being produced. MPV gives your doctor information about platelet production in your bone marrow.
- Mean corpuscular volume (MCV) is a measurement of the average size of your RBCs. The MCV is elevated when your RBCs are larger than normal (macrocytic), for example in anaemia caused by vitamin B12 deficiency or folic acid deficiency. When the MCV is decreased, your RBCs are smaller than normal (microcytic), which may indicate iron deficiency anaemia, inflammation or occasionally thalassaemias.
- Mean corpuscular haemoglobin (MCH) is a calculation of the amount of oxygen-carrying haemoglobin inside your RBCs. Since macrocytic RBCs are larger than either normal or microcytic RBCs, they would also tend to have higher MCH values.
- Mean corpuscular haemoglobin concentration (MCHC) is a calculation of the concentration of haemoglobin inside the RBCs. Decreased MCHC values (hypochromia) are seen in conditions where the haemoglobin is abnormally diluted inside the red cells, such as in iron deficiency anaemia, long standing inflammation or thalassaemia. Increased MCHC values (hyperchromia) are seen in conditions where the haemoglobin is abnormally concentrated inside the red cells, such as in hereditary or autoimmune spherocytosis.
- Red cell distribution width (RDW) is a calculation of the variation in the size of your RBCs. In some anaemias, such as iron deficiency or pernicious anaemia, the amount of variation (anisocytosis) in RBC size (along with variation in shape – poikilocytosis) causes an increase in the RDW.
When is it requested?
The FBC is a very common test used to screen for, help diagnose, and to monitor a variety of conditions. Many patients will have baseline FBC tests to help determine their general health status. If they are healthy and they have cell populations that are within normal limits, then they may not require another FBC until their health status changes or until their doctor feels that it is necessary.
If a patient is having symptoms associated with anaemia, such as fatigue or weakness, or has an infection, inflammation, bruising, or bleeding, then the doctor may order a FBC to help diagnose the cause. Significant increases in WBCs may help confirm that an infection is present and suggest the need for further testing to identify its cause. Decreases in the number of RBCs (anaemia) can be further evaluated by changes in size or shape of the RBCs to help determine if the cause might be decreased production, increased loss, or increased destruction of RBCs. A platelet count that is low or extremely high may confirm the cause of excessive bleeding or clotting.
Many conditions will result in increases or decreases in the cell populations. Some of these conditions may require treatment, while others will resolve on their own. Some diseases, such as cancer (and chemotherapy treatment), can affect bone marrow production of cells, increasing the production of one cell at the expense of others or decreasing overall cell production. Some medications can decrease WBC counts, and some vitamin and mineral deficiencies can cause anaemia. The FBC test may be ordered by the doctor on a regular basis to monitor these conditions and drug treatments.
What does the test result mean?
The following table explains what increases or decreases in each of the components of the FBC may mean.
Components of the FBC
| Test | Name | Increased/decreased |
|---|---|---|
| WBC | White Blood Cell | May be increased with infections, inflammation, cancer, leukaemia; decreased with some medications, some autoimmune conditions, some viral or severe infections, bone marrow failure, enlarged spleen, liver disease, alcohol excess and congenital marrow aplasia (marrow doesn't develop normally) |
| % Neutrophil |
Neutrophil/Band/ANC |
This is a dynamic population that varies somewhat from day to day depending on what is going on in the body. Significant increases in particular types are associated with different temporary/acute and/or chronic conditions. An example of this is the increased number of lymphocytes seen with lymphocytic leukaemia. For more information, see Blood Film and WBC |
| Lymphs | Lymphocyte | |
| % Mono |
Monocyte | |
| % Eos |
Eosinophil |
|
| % Baso |
Basophil |
|
| RBC | Red Blood Cell |
Decreased with anaemia; increased when too many made and with fluid loss due to diarrhoea, dehydration, burns |
| Hb | Haemoglobin | Mirrors RBC results |
| Hct | Haematocrit | Mirrors RBC results |
| MCV | Mean Corpuscular Volume | Increased with B12 and Folate deficiency, liver disease, underactive thyroid, pregnancy, alcohol excess, some bone marrow disorders; decreased with iron deficiency, longstanding inflammatory disorders and thalassaemia |
| MCH | Mean Corpuscular Haemoglobin | May be low in iron deficiency, inflammatory conditions and thalassaemia |
| MCHC | Mean Corpuscular Haemoglobin Concentration | Helps with interpretation of MCH |
| RDW | RBC Distribution Width | Increased RDW indicates abnormal variation in RBC size. Can indicate iron deficiency or bone marrow disorders. |
| Platelet | Platelet | Increased numbers of platelets occur with bleeding, inflammation, bone marrow disorders and in patients with absent or underactive spleens. Decreased numbers are associated with immune conditions such as ITP and SLE, vitamin deficiencies, some drugs (especially chemotherapy), alcoholism, liver disease, enlarged spleens, bone marrow disorders and with some rare inherited disorders (such as Wiskott-Aldrich, Bernard-Soulier) |
| MPV | Mean Platelet Volume |
Vary with platelet production; younger platelets are larger than older ones |
Is there anything else I should know?
Common Questions
Ask a Laboratory Scientist
Article Sources
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
