A haemocytometer is a device which contains a small chamber which can be filled with fluid. The top of the small chamber is transparent and has a microscopic grid etched into it. Using a microscope it is possible to see through the gird and count any visible particles which are inside the chamber (suspended in the liquid). Because the depth of the chamber and the size of the grid are known, it is then possible to calculate the concentration of particles in the liquid. This article will discuss how a haemocytometer is used to count cells in blood.
Firstly, the chamber on the haemocytometer must be filled with the correct volume of properly homogenised (mixed) blood. This can be done using a capillary tube, and allowing the blood to flow into the chamber by capillary action. Measuring the blood using a pipette is not necessary because the size and shape of the chamber will cause it to fill correctly.
Secondly, the scientist/technician must use a microscope to look through the grid on the chamber and count cells. The scientist/technician can count as many cells as he or she feels is necessary in order to obtain accurate results. However, a few things are imperative: the first is that the section of the grid which is counted truly represents the entire grid. For example, it would be inappropriate to count a section of the grid which, by chance, has more cells than the other parts of the grid; the second is that the scientist/technicians knows how much of the grid he or she counted. This information is needed in order to do the calculations, which is the next step.
It is imperative that the scientist/technicians understands what volume of liquid each square of the grid represents. Typically, the chamber is 0.1mm deep and the grid consists of several 1mm squares. Thus, one of these squares represents 100nL of blood. Thus, if the scientist counts 50 cells (for example) in a 1mm square, he or she can conclude that the concentration of cells is 50 per nL, and the scientist can then express that in whatever units are appropriate.
For simplicity and practicality, the 1mm squares mentioned in the above paragraph are also divided into even smaller squares. This is because there may be too many cells to count all the cells in a large square, and it may be more appropriate to count the cells in a smaller square – the scientist/technician will simply need to correct for this when doing their calculations.
My final point is that blood contains many types of cells. Without a stain red cells (erythrocytes), white cells (leukocytes) and platelets are typically easy to distinguish from each other and thus any of these cell lines can be counted using the haemocytometer. Of course cells which cannot be easily recognized cannot be counted using this method. For example, it may not be easy to distinguish one type of white cell from another type of white cell. It may be possible to overcome this issue by mixing the blood with an appropriate stain before adding it to the chamber, but obviously any dilution must be subsequently corrected for during the calculations. Modern routine laboratories do not encounter this problem because they do not count cells in blood using this method – there are several alternative methods. Having said that, some modern routine pathology laboratories do those a haemocytometer to count cells in urine. They typically count red cells, white cells, and epithelial cells (and they may also count other cells and microscopic structures if they are present).
