The haematology analyser is a laboratory machine that performs automated cell counts on an aliquot from the EDTA tube.
A red cell count is done by virtually all haematology analysers, even the ancient machines that still perform only a 3-cell differential based on coulters principle (which I am sure are still in use somewhere). If the red count is missing and you want it, ring the lab: the machine certainly calculated it.
First in class: the Coulter counter
The very first analysers were simple Coulter counters based on the changes in electrical current through as a cell moves through an aperture within an electrolyte solution.
Their true utility came from the relationship between the size of the cell and the size of the current drop. The counter could now identify erythrocytes, thrombocytes, and leukocytes and produce a 3-cell differential.
From 3-cell to 5-cell
It is quickly apparent why the 3-cell differential falls short; it fails to provide detailed counts for the leukocytes. Realistically, the absence of a basophil count solves the issue of detecting basophilia and being unable to explain it…
Nonetheless, the analysers have moved on to produce 5- or even 7-cell differentials previously not possible with simple the simple Coulter principle
Modern haematology analysers
Having touched on the novel idea of a 7-cell leukocyte differential one would be forgiven for thinking none of this is possible without something like modern flow cytometry; one would be right.
Take, for example, some of the newer analysers by Beckman Coulter or Boule. They are really two machines (or three if the spectrophotometer is included): Coulter counter and flow cytometer.
It seems that for many of the modern haematology workstations or standalone machines, a routine CBC involves some kind of contemporary cytometry in addition to electrical impedance.
The flow cell Coulter counter
Having discussed the classical application of Coulter’s principle to an aperture, we need to get realistic and acknowledge that, like most things, its not that simple.
Let us consider the “aperture”: it could be any shape or form. If it were a long tube with an electrode at each end it would function the same. That is the principle of the flow cell format.