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Blood counting machines work by sampling blood, and sucking a standard amount through a small tube. Within this tube, 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.
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 the may be misidentified by an automated counter. It is however subject to human bias, and has a much smaller sample size. Additional factors, such as the quality of the blood film, also play a greater part.
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.
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