COULTER COUNTER

Typical Number in Hospital: 3

Cost Bands: 5,6

References: 9

This is a trade name for a particle counter which calculates a number of characteristics of blood samples, and is used in the haematology department. The essential element in the device is a cell counter (haemocytometer) which causes blood cells to pass through a small aperture through which an electric current is being passed between electrodes each side of the aperture. As a cell passes through there will be a transient change in the electrical resistance since the resistance of a blood cell will be higher than the electrolyte in which it is suspended.

There are various types of Coulter counter classified according to the number of analyses performed. A useful example is the model 'S' in which the sample of blood is diluted in a ratio of 1:224 and then split into two separate channels, one for determining the white cell count and haemoglobin content, and the other for determining the red cell count. In the first channel a lysing agent is added to break down the red blood cells (from which the haemoglobin content is calculated by spectrophotometric methods) and the white cells are counted in an aperture device as described above, usually with three separate channels which should all agree within a defined margin for the final result. The second channel undergoes further dilution because of the high red blood cell count and is then fed to a similar three aperture device for counting the cells. The model 'S' contains a calculating module which uses these results to determine a number of commonly used indices of blood composition including the mean corpuscular volume (MCV), haematocrit, mean corpuscular haemoglobin (MCH), mean corpuscular haemoglobin concentration (MCHC), etc.

There are other types of haemocytometer which utilize optical detection of the passage of cells through the gate such as the Fisher autocytometer.