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Automated Cell Counters

By Edited Nov 13, 2013 0 0

Counting of cells is a procedure that is intended to determine the number of individual cells of each type that are included in a unit volume of blood (usually, in 1 mm3).

All cell counts have 3 stages described below.

    Blood dilution.

    Counting the number of cells.

    Mathematical calculation of the amount of cells in 1 mm3.

Cellular counts can be performed using manual methods (counts in hemocytometer chamber) or automated methods (automated counters). The first option has a the wide margin of (human) error. It is also quite time-taking.

The hematology analyzers or automated cell counters are now the choice in modern laboratories. The margin of error obtained by manual methods, which hovers around 20%, is greatly diminished with the development of these devices, which present an error margin of around 1%.

As it will be seen throughout this article, automated cell counters are not limited to carrying out the cell count of the different cellular components. They usually provide the results of each and every one of the constituents of the blood.

Components of the automated counter

Diluter

Decreases the blood concentration to the appropriate level for the operation of the counter.

It usually dilutes the sample only slightly for the leukocyte count (eg, 1/300 for Microsot ABX) and much more for an erythrocyte count (for example, 1:20,000 for Microsot ABX). Dilution will vary depending on the particular counter, but this will be independent on the technician.

Compressor: vacuum

Provides the necessary vacuum pressure to carry the blood, which has been suitably diluted, to the measuring device.

Measuring device

This is the place where the chamber to count cells really is. Its design depends on the counting method used by each model.

It may be based on a detection system for measuring the cell's impedance or on optical systems.

In most advanced electronic counters, a ceramic valve divides the sample into different measuring chambers. As each chamber is specifically designed for counting a different cell type (red cells, platelets or leukocytes) or for the determination of hemoglobin.

Transducer

Transforms the signals from the measurement device into electrical pulses. It is usually a photomultiplier.

Discriminator

Segregates the electrical pulses generated by each of the types of blood cells.

Processor

Collects the data, processes it and displays it on a screen.

Recorder

It prints the results obtained.

Automated cell counting: methods

Here are the most common methods used to count cells in automated cell counters.

Electrical resistance method or impedance.

It was discovered by Coulter in year 1956 and used in the first automated cell counters. It is based on the following: while blood cells have low electrical conductivity, the liquid diluent is a good conductor of electricity.

    The measuring device consists of a small hole through which the diluted blood passes. There is an electrode at the entrance and one at the exit. An electric current is passed through the same hole.

    When the hole is traversed only by diluting fluid, the electrical resistance measured by the electrodes is low and constant, but when the hole is traversed by a blood cell, there is an increase in electrical resistance and a change in the potential between the electrodes.

    The count of electrical signals indicate the number of cells present in the blood and the amplitude of these signals is directly proportional to the volume of cells. Thus it is possible to identify the cells and obtain the count.

Scattering method or light diffraction.

Dark field method

The measuring device consists of a capillary through which the diluted blood circulates and which is traversed by a halogen light beam.

When cells do not pass along the capillary, the beam encounters a non-sensitive area (darkfield disk), but if a cell passes through the capillary, the rays of the light beam are scattered out of the disc, and are captured by a photodetector.

The number of detected light signals indicates cell numbers present in the blood, and the light scattering intensity produced by each of them is directly proportional to the size and cell content.

Laser beam method

The measuring device comprises a detector located behind a capillary where a continuous flow of diluted blood circulates, it is therefore a flow cytometer. A laser beam passes through the capillary and is directed towards the detector.

When cells do not pass along the capillary, the laser beam encounters the detector, but if a cell passes through the capillary, the laser beam is intercepted by it and stops pointing on the detector.

The interference number indicates the number of cells present in the blood, and the degree of interference that each cell produces is directly proportional to its size (forward scatter).

In these meters, the laser light may be, for example, a helium-neon laser polarized vertically to a wavelength of 632.8 nm (the counters used in CELL-DYN 3000 and 3500 from Abbott Laboratories).

Parameters that an automated counter can give

The modern automated counters are able to determine more than 50 parameters. Among these are the following:

    Number of red blood cells per mm3 of blood.

    Reticulocyte percentage.

    Number of leukocytes per mm3 of blood.

    Number of platelets per mm3 of blood.

    Hematimetric indices (erythrocytes, reticulocites, leukocytes and platelets).

    Hematocrit value.

    Hemoglobine concentration in blood.

    Erythrocyte sedimentation rate.

All these parameters are the analyses included in the hemogram or also called basic blood profile.

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