The full blood count (FBC) is one of the most commonly requested 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.

The FBC is a very common test used to screen for, help diagnose, and to monitor a variety of conditions.

If a patient is having symptoms associated with , 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 WBC's may help confirm that an infection is present and suggest the need for further testing to identify its cause. Decreases in the number of RBC's (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 that is low or extremely high may need further investigation.

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 requested by the doctor on a regular basis to monitor these conditions and drug treatments.

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 count (WBC) is a count of the actual number of white blood cells per volume of blood. Both increases and decreases can be significant.

White Cell Count 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, lymphocytes, monocytes, eosinophils and basophils.  

Red Blood Cell Count (RBC) 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 bone marrow 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.

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  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 vitamin B12 deficiency, the amount of variation (anisocytosis) in RBC size (along with variation in shape – poikilocytosis) causes an increase in the RDW.