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 abnormal conditions of excess bleeding or clotting. 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. When the MCV is decreased, your RBCs are smaller than normal (microcytic), such as is seen in  iron deficiency anaemia or 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 and in thalassaemia. Increased MCHC values (hyperchromia) are seen in conditions where the haemoglobin is abnormally concentrated inside the red cells, such as in hereditary spherocytosis, a relatively rare congenital disorder.
• Red cell distribution width (RDW) is a calculation of the variation in the size of your RBCs. In some anaemias, such as pernicious anaemia, the amount of variation (anisocytosis) in RBC size (along with variation in shape – poikilocytosis) causes an increase in the RDW.

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.

NOTE: A standard reference range is not available for this test. Because reference values are dependent on many factors, including patient age, gender, sample population, and test method, numeric test results have different meanings in different labs. Your lab report should include the specific reference range for your test. Lab Tests Online strongly recommends that you discuss your test results with your doctor. For more information on reference ranges, please read Reference Ranges and What They Mean.

 

Test	Name	Increased/Decreased
WBC	White Blood Cell	May be increased with infections, inflammation, cancer, leukaemia; decreased with some medications (such as methotrexate), 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/Seg	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 Smear and WBC.
% Lymphs	Lymphocyte
% Mono	Monocyte
% Eos	Eosinophil
%Baso	Basophil
Neutrophil	Neutrophil/Band/Seg
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
Hgb	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 diorders and thalassaemia
MCH	Mean Corpuscular Haemoglobin	Mirrors MCV results
MCHC	Mean Corpuscular Haemoglobin Concentration	May be decreased when MCV is decreased; increases limited to amount of Hgb that will fit inside a RBC
RDW	RBC Distribution Width	Increased RDW indicates mixed population of RBCs; immature RBCs tend to be larger
Platelet	Platelet	Decreased or increased with conditions that affect platelet production; decreased when greater numbers used, as with bleeding; decreased with some inherited disorders (such as Wiskott-Aldrich, Bernard-Soulier), with Systemic lupus erythematosus, pernicious anaemia, hypersplenism (spleen takes too many out of circulation), liver disease, leukaemia, and some drugs including chemotherapy
MPV	Mean Platelet Volume	Vary with platelet production; younger platelets are larger than older ones