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.
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.
A complete blood count will normally include:
A complete blood count with differential will also include:
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.