A full blood count (FBC) or complete blood count (CBC) is a test requested by a doctor or other medical professional that gives information about the cells in a patient's blood.

Automated blood count

Blood for a FBC is usually taken into an EDTA tube to stop it from clotting. The blood is well mixed (though not shaken) and put through a machine. The machine, called an automated analyser, counts the numbers and types of different cells within the blood. The machine prints out, and/or sends to a computer, the results.

Blood counting machines work by sampling blood, and sucking a standard amount through narrow tubing. Within this tubing, there are sensors that count the number of cells going through it, and can identify the type of cell.

The two main sensors used are light detectors, and electrical impedance.

Because an automated cell counter samples and counts so many cells, it gives a very precise estimate. However, with certain abnormal cells in the blood, they may be identified incorrectly, and not be as accurate as a manual count.

Automated blood counting machines include the Sysmex XE-2100 and the Abbott Cell-Dyn range.

Manual blood count

Counting chambers that hold a specified volume of diluted blood (as there are far too many cells if it is not diluted) are used to calculate the number of red and white cells per litre of blood.

To identify the numbers of different white cells, a blood film is made, and a large number of white cells (at least 100) are counted. This gives the percentage of cells that are of each type. By multiplying the percentage, with the total number of white blood cells, an estimate of the absolute number of each type of white cell can be obtained.

Manual counting has the advantage in that it can identify blood cells that may be misidentified by an automated counter. It is, however, subject to human error , and has a much smaller sample size. Additional factors, such as the quality of the blood film, also play a greater part.

Results from a blood count

A complete blood count will normally include:

Red cells

• Total red blood cells - The number of red cells is given as an absolute number per liter.
• Hemoglobin - The amount of hemoglobin in the blood, expressed in grams per liter. (Low hemoglobin is called anemia.)
• Hematocrit or packed cell volume (PCV) - This is the fraction of whole blood volume that consists of red blood cells.
• Mean cell volume (MCV) - the average volume of the red cells, measured in femtolitres. Anemia is classified as microcytic or macrocytic based on whether this value is above or below the expected normal range. Other conditions that can affect MCV include thalassemia and reticulocytosis.
• Mean cell hemoglobin (MCH) - the average amount of hemoglobin per red blood cell, in picograms.
• Mean cell hemoglobin concentration (MCHC) - the average concentration of hemoglobin in the cells.
• Red cell distribution width (RDW) - a measure of the variation of the RBC population

White cells

• Total white blood cells - All the white cell types are given as a percentage and as an absolute number per litre.

A complete blood count with differential will also include:

• Neutrophil granulocytes - May indicate bacterial infection.
• Lymphocytes - Higher with some viral infections such as glandular fever. Also raised in lymphocytic leukaemia CLL.
• Monocytes - May be raised in bacterial infection
• Eosinophil granulocytes - Increased in parasitic infections.
• Basophil granulocytes

A manual count will also give information about other cells that are not normally present in peripheral blood, but may be released in certain disease processes.

Platelets

• Platelet numbers are given, as well as information about their size and the range of sizes in the blood.