AUTO ANALYSER

Typical Number in Hospital: 2

Cost Bands: 6,7

References: 9

A large part of the work of the clinical chemistry laboratory in a hospital is to estimate the concentrations of various ions, molecules and enzymes in samples of body fluids taken from the tissues, blood, urine, cerebro-spinal fluid, etc. For many of these tests the analysis procedure consists of splitting the sample for the number of separate investigations requested, diluting these sub-samples, adding the reagents, heating, mixing, etc. and then analysing in a colorimeter, flame or spectrophotometer or other detector.

Since these tests are required in large numbers automated apparatus has become available which takes in samples at one end and prints the results at the other. There are three main types of auto analyser each with its own advantages and disadvantages.

1. continuous flow analysers. These employ an intrigueing system by which the subdivided sample and a quantity of reagent are drawn through a long narrow plastic tube to the detection device (colorimeter or flame photometer). The necessary chemical reaction takes place during the time the bolus of fluid is passing along the tube, and it may be accelerated by heating if this is necessary for the particular chemical process involved. After a sample and quantity of reagent have been fed into the tube an air bubble is introduced followed by water, followed by another air bubble before the next sample. With this arrangement each sample with reagent is allowed to pass along the tube without mixing with any other in the same continuous flow stream. The meniscus of the bubble serves to clean the inside of the tube to prevent contamination of the samples. The Technicon SMA (sequential multiple analyser) series of auto analysers use continuous flow.

2. centrifugal analysers. In these the sample and reagent are pipetted into wells (e.g. 30 of each) in the rotor of a centrifuge. When the centrifuge is started each sample and reagent pair is spun down into a cuvette on the edge of the rotor where they pass between the light source and photometer of a colorimeter. An electronic system identifies the photometric results to the correct sample and reagent cuvette, and can monitor the progress of the reaction in each cuvette. The end-point of each reaction can be identified and used in the same way as the continuous flow analyser, but it is also possible to identify reaction rate which is useful in estimating enzyme concentration. An example of a centrifugal analyser is the Centrifichem made by the Union Carbide Corporation (now marketed under the name Encore).

3. Discrete sample analyser. In these the sample is diluted and separated into a number of sub-samples appropriate to the number of tests requested. These are then mixed with reagents and the necessary time or heat treatment given before passing to a detector device to monitor the end point. Although this system appears more clumsy than the continuous flow analyser its advantage is that the machine can be programmed to perform only those tests which are requested, so saving on expense. Also it may be possible to have a bigger battery of tests available, though not all on need to be performed on the same sample.