The FBC is used as a broad screening test to check for such disorders as anaemia (decrease in red blood cells or haemoglobin), infection, and many other diseases and to monitor treatment. It is actually a group of tests that examine different parts of the blood. Results from the following tests provide the broadest picture of your health:

• White blood cell (WBC) count measures the total number of white blood cells per volume of blood. Both increases and decreases can be significant. A typical WBC in an adult is 4 - 11 x 10⁹/L (four to eleven thousand million per litre of blood).
• White blood cell differential: looks at the types of white blood cells present. There are five main 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. Immature forms may also be present in certain conditions. These include metamyelocytes, myelocytes, promyelocytes and blast cells. The individual counts can be reported as either absolute counts and/or as a percentage of the total.

Red blood cell tests:

• Red blood cell (RBC) count: the number of red blood cells per litre of blood. Both increases and decreases can point to abnormal conditions. Red blood cells are reported as billions per litre (e.g. 4.25x10¹²/L).
• Haemoglobin: is the iron containing oxygen-carrying protein in the red cells. Haemoglobin values are higher in males than in females and results are reported in units of g/L or g/dL (e.g 125 g/L is equivalent to 12.5 g/dL).
• Haematocrit: the proportion of the total blood volume that consists of red blood cells. It is reported as a ratio.
• Mean cell volume (MCV): a measurement of the average size of your red blood cells (RBC). 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 cell 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 cell 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.

Platelet tests:​

• Platelet count: 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. Platelets are reported as thousand millions per litre (150 - 400 x 10⁹/L).

FBC results that are outside the reference ranges may indicate the presence of disease or conditions. Other tests, such as a blood film, are performed to help determine the cause of the abnormal results.

 

An FBC is usually requested as a routine blood test. It is also requested for a variety of other more specific situations. These can include:

• to determine how severe a blood loss is
• to help diagnose infection
• to help diagnose diseases such as leukaemia or anaemia
• to monitor the response to some types of drug or radiation treatment
• to investigate a history of abnormal bleeding or clotting

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 (tiredness) or weakness, or has an infection, inflammation, bruising, or bleeding, the doctor may order a FBC to help identify the cause. Significant increases in WBCs may help confirm that an infection is present and suggest the need for further testing to identify it. 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 that it is caused by excessive bleeding or clotting which can be associated with disease of the bone marrow.

Many conditions 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.

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, allergies, cancer, myeloproliferative neoplasm, leukaemia; decreased with some medications (such as methotrexate), some autoimmune conditions, some viral or severe infections, bone marrow failure, lymphoma, enlarged spleen, liver disease, alcohol excess and congenital marrow aplasia (marrow doesn't develop normally), diseases of the immune system (e.g. HIV/AIDS)
%Neutrophils	Neutrophil/PMNs/Neuts	This is a dynamic population that varies somewhat from day to day depending on what is going on in the body. Significant increases are associated with different temporary/acute and/or chronic conditions.  See Blood film examination and WBC.
%Lymphocytes	Lymphocyte	An increase in the number of lymphocytes may be seen in a variety of conditions including viral illness (e.g. cytomegalovirus (CMV), Epstein-Barr virus (EBV), certain drug treatments and chronic lymphocytic leukaemia. Decreases in lymphocytes  may be seen in autoimmune disorders (eg lupus and rheumatoid arthritis), infections (eg HIV, viral hepatitis), bone marrow damage.
%Eosinophils	Eosinophil	The number of eosinophils may be increased in allergies such as hay fever, asthma and parasitic infections.
%Monocytes	Monocyte
%Basophils	Basophil
RBC	Red Blood Cell	Decreased with anaemia; due to acute or chronic bleeding, RBC destruction, bone marrow disorders. Increased when too many made (polycythaemia) and with fluid loss due to diarrhoea, dehydration, burns, living at high altitude.
Hb	Haemoglobin	Mirrors RBC results
PCV/Hct	Haematocrit	Mirrors RBC results
MCV	Mean cell volume	Increased (larger than normal RBCs - macrocytic) with vitaminb B12 and folate deficiency ; Decreased (smaller than normal RBCs - microcytic) with iron deficiency and thalassaemia
MCH	Mean cell haemoglobin	Mirrors MCV results
MCHC	Mean cell haemoglobin concentration	May be decreased when MCV is decreased; increases limited to amount of Hb that will fit inside a RBC
RDW	RBC distribution width	Increased RDW indicates mixed population of RBCs. This may be due to an increase in the numbers of immature RBCs which tend to be larger. It can also be seen following a blood transfusion or iron therapy.
Platelet	Platelet	Increased number of platelets occur after bleeding, inflammation and surgery. Decreased numbers are associated with some inherited disorders (such as Wiskott-Aldrich, Bernard-Soulier), with Systemic lupus erythematosus, pernicious anaemia, hypersplenism (spleen takes too many out of circulation), leukaemia, and chemotherapy
MPV	Mean platelet volume	Vary with platelet production; younger platelets are larger than older ones. A low MPV indicates that a condition is affecting the production of platelets by the bone marrow. A high MPV indicates that a condition is causing an overproduction and rapid release of platelets into circulation

About Reference Intervals

A number of conditions can be detected. Some examples include:

• Anaemia
• Autoimmune disorders
• Bone marrow disorders
• Infections
• Haemoglobin abnormalities
• Leukaemia
• Sickle cell disease
• Thalassemia

Patients who have a keen interest in their own health care frequently want to know what they can do to change their WBCs, RBCs, and platelets. Unlike 'good' and 'bad' cholesterol, cell populations are not generally affected by lifestyle changes unless the patient has an underlying deficiency (such as vitamin B12 or folate deficiency or iron deficiency). There is no way that a patient can directly raise the number of their WBCs or change the size or shape of their RBCs. Addressing any underlying diseases or conditions and following a healthy lifestyle will help optimise your body's cell production and your body will take care of the rest.