DC sheath flow detection method

Sysmex analysers use the DC sheath flow detection method to count red blood cells and platelets, RBC and PLT. A portion of blood is separated from the aspirated whole blood and mixed with the diluent in a pre-set ratio.

Of this dilution a defined amount is sent to the detection chamber and passed through a small opening, known as the aperture. There are also electrodes on each side of the aperture – and direct current passes through these electrodes. The direct current resistance between the electrodes changes as blood cells suspended in the diluent pass through the aperture. This resistance causes an electrical pulse change proportional to the size of the blood cell.

These electrical data are converted into graphical displays of volume distribution curves, or histograms.

Once the cells leave the sample nozzle exit they are surrounded by a sheath flow of diluent. Here, they are aligned and moved to the centre of the orifice. This reduces interference errors and the possibility of abnormal cell pulse detection, which could be caused by cells passing through the transducer off-centre. As soon as the cells have passed the orifice, they are seized by another, inverse flow and immediately led to the drain. This prevents renewed circulation and  a change in the platelet count.