The Sysmex XE -2100 hematology system is a flow cytometer that uses the principle of electrical impedance associated with an optical system based on a beam of monochromatic light emitted by a semi-conductor laser.

The counts and volumetric measurements of RBCs and PLTs are obtained with the impedance, or resistive, detection method. The system uses hydrodynamic focusing, by which cells are collected in flow of diluent under pressure, aligned in single row and directed toward the center of the orifice of the transducer. This procedure reduces the error of coincidence and the production of abnormal pulses, generated when cells do not pass exactly in the center of the counting orifice, or in the case of recirculation in the detection zone. Reticulocyte count is obtained after addition of a polymethine fluorescent dye, which penetrates through the cell membrane: reticulocytes are separated from mature erythrocytes thanks to their residual RNA content. Nucleated cells, on the other hand, are identified and counted after fluorescent staining of their nuclear DNA and cytoplasmic RNA, using signals of light scatter and forward/side fluorescence. Reticulocytes are subdivided into 3 fractions (HFR, MFR and LFR) as a function of their different amount of residual RNA. A second optical PLT count (Plt-O) is determined from the reticulocyte scattergram. In samples with abnormal distribution of PLTs, the value of Plt-O is automatically reported: it is in fact more reliable in the presence of large PLTs or severely microcytic erythrocytes. Moreover, the value of Plt-O has been described as less influenced by the presence of cryoglobulins, being determined at a temperature of 41°C.

The separation of the different WBC populations for the determination of leukocyte differential count is obtained by using three different signals: forward scatter (FSc), side scatter (SSc) and side fluorescence (SFL). The intensity of forward scatter is proportional to the volume of the cell, while side scatter depends on the characteristics of cell content, such as nucleus and granules. Side fluorescence is proportional to the amounts of DNA and RNA present in the cell. These measurements are carried out ​​in different analytical channels to obtain a complete differential leukocyte count, flags of morphological abnormalities and the count of erythroblasts and immature granulocytes. The majority of information is obtained in the Diff channel, the most useful for the characterization of leukocytes: the greater the complexity of the nucleus and the number of granules, the higher is the intensity of side scatter signal. The side fluorescence signal depends on the amount of nucleic acids contained in the cell: the greater the amount of nucleic acids, the greater will be the signal intensity. Basophils are counted in the specific WBC/Baso channel, together with the total count of leukocytes. In the additional NRBC channel, circulating erythroblasts are identified and counted: here the polymethine fluorescent dye stains nucleic acids of WBCs and erythroblasts, thus clearly separating the two cell populations on the basis of different fluorescence intensity.

Flags for the presence of abnormal cells, such as atypical lymphocytes, blasts and immature granulocytes, are derived from the analysis of all information collected in these analytical channels, in combination with that obtained in the IMI channel, specifically dedicated to the recognition and counting of immature and progenitor cells.