Automated analyser

From Academic Kids

An automated analyser is a laboratory machine designed to process a number of samples quickly, with minimal human assistance.

Different methods of putting samples through have been invented, but usually involve placing test tubes of sample into racks, which can be moved along a track, or circular carousels that rotate to make the sample available. To protect the health and saftey of laboratory staff many analysers feature closed tube sampling to prevent workers from direct exposure to samples.

Samples can be processed in batches, or continuously.

Contents

Routine biochemistry analysers

These are machines that process the bulk of the samples going into a hospital or private medical laboratory. And the results should be out as quickly as possible. There will often be a method that can get urgent specimens moved more quickly through.

The types of tests required are often enzyme levels (such as many of the liver function tests), ion levels (e.g. sodium and potassium), and other tell-tale chemicals (such as albumin or creatinine).

Simple ions are done with ion selective electrodes, that let one type of ion through, and measure voltage differences. Enzymes are measured by the rate they change one coloured substance to another; the results for enzymes are given as an activity, not a concentration of enzyme. Other tests use colorimetric changes to determine the concentration.

Turbidity (as created when an antibody reacts with a test compound) can also be measured with these machines.

Examples of these types of machines are:

Immuno-based analysers

Because many substances (such as hormones or drugs) have no colour, and cannot cause another substance to change colour, antibodies must be used to detect them.

The concentration of these compounds is often too low to cause a measurable increase in turbidity when bound to antibody, so other, more specialised, methods must be used.

Examples of these analysers include:

Haematology analysers

These are used to perform full blood counts, erythrocyte sedimentation rates (ESRs) or coagulation tests.

Cell counters

Cell counting machines sample the blood, and analyse its cell populations using both electrical and optical techniques. Electrical analysis involves passing a dilute solution of the blood through an aperture across which an electrical current is flowing. The passage of cells through the current changes the impedance between the terminals (the Coulter principle}. A lytic reagent is added to the blood solution to selectively burst the red cells, leaving only white cells and platelets intact, then the solution is passed through a second detector. This allows the a differential count of RBCs against WBCs and platelets to be obtained. The Platelet count is easily separated from the WBC count by the smaller impedance spikes they produce in the detector due to their small volume.

Optical detection is utilised to gain a differential count of the populations of white cell types. A dilute suspension of cells is passed through a flow cell, which passes cells one at a time through a capillary tube past a LASER beam. The reflectance, transmission and scattering of light from each cell is analysed by sophisticated software giving a numerical representation of the likely overall destibution of cell populations.

Reticulocyte counts can be performed by many analysers, but are just as often carried out manually by staining blood in a Merrett tube and performing a cell count under the microscope. Some analysers have a modular slide maker which is able to both produce a blood film of consistent quality and stain the film.

Examples of full blood count machines are:

Coagulometers

Automated coagulation machines or Coagulometers measure the ability of blood to clot by performing several types of tests including clotting screens, INRs, lupus anticoagulant screens and factor assays.

Coagulometers require blood camples that have been taken in tubes containing trisodium citrate as an anticoagulant. These are used rather than EDTA anticoagulated tubes as the mechanism behind the anticoagulant effect is reversible by the addition of calcium ions to the solution. Depending on the test different substances can be added to the blood plasma to trigger a clotting reaction. The progress of clotting is measured optically by measuring the absorbance of a particular wavelength of light by the sample and how it changes over time.

Coagulation machines include:

Other Haematology Apparatus

Automatic ESR readers, while not strictly analysers, hold a rack of samples for an hour, then after an hour determine how far the red cells have fallen, by detecting levels with light beams.

As ESR tests become less popular they are being replaced by Plasma Viscosity tests. The advantage of this test over ESR is that less variables can affect the result so it can give a more direct impression of the plasma protein content of a sample. The analysers commonly work by drawing a small sample of plasma through a narrow capillary using a constant pressure and measuring the time taken for the sample to move a known distance.

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