199 - Hematology flow analyzers: history of continuously evolving technology. Part 1: the impedance method
Autor(s): Buttarello M.
Issue: RIMeL - IJLaM, Vol. 2, N. 3, 2006 (MAF Servizi srl ed.)
Automated flow cytometers that provide dozens of parameters from a small sample of blood can be found in clinical laboratories the world over.
The methods used are based on two fundamental principles: electrical resistance or impedance and optical analysis. The principle of impedance was formally invented by W. Coulter in 1949 and enabled cytometers to count and measure the volume of cells analyzed.
This method was made more reliable and its application was extended thanks to many improvements: from hydrodynamic focusing to the correction for coincidence, from the recognition and elimination of abnormal signals to the use of lysing agents which are selective for various leukocyte populations.
The production in 1968 of the Coulter Model S, with sampling valve and automatic correction for coincidence, revolutionized the activity of hematological laboratories.
A similar method, based on capacitance rather than resistance measurements, was developed in the 1950s in Japan and constituted the basis for the first analyzers commercially produced by Toa ( now Sysmex ).
A further extension of Coulter’s principle consisted of the contemporary use of low and high frequency current to examine different cell characteristics: size in the first case and composition and internal structure in the second. These biparametric measurements were applied commercially starting in 1986 with the VCS transducers produced by Coulter and in 1988 with the NE series of analyzers produced by Sysmex.