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SUMMARY.After some brief remarks on counting chambers, references to the ICSH-recommended haemoglobin-determination are given. The microhaematocrit of normal blood is advocated as a potential routine calibration method. Comments are given on discrepancies between centrifugal and flow haemocytometry haematocrits of abnormal and artificial bloods. Flow haemocytometry instruments are classified into analogue and digital instruments or into electrical and optical instruments. Their hydrodynamic properties are discussed. The principles and problems of electrical and optical cell counting and sizing are dealt with. The importance of the refractive index and of flow-induced cell shape changes for the MCV determinations is stressed. It is argued that MCV and haematocrit values are exaggerated at both low and high values and consequently MCHC is erroneously constant. Various prevailing red cell distribution width (RDW) and platelet distribution width (PDW) definitions bring about considerable confusion. The major features of the counting and sizing of white blood cells and platelets are described.

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