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. 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. 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: , , , , and . Immature forms may also be present in certain conditions. These include metamyelocytes, myelocytes, promyelocytes and blast cells.
- 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 (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 (125 g/L is equivalent to 12.5 g/dL).
- Haematocrit: the proportion of space red blood cells take up in the blood. It is reported as a ratio.
- 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).
- Mean corpuscular 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 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 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.
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.