Description of Device Types

Different types of device/assay are available to detect serological markers of infection. A simple description of the different types follows:

Enzyme Immunoassays (EIAs or ELISAs)

Simple / Rapid Test Devices (S/RTDs)

Line Immunoassays (LIAs)

Automated Random Access Analysers

Open Serology Processors

Agglutination Assays

Immunoassays are based on the principle that antigens from microbes will stimulate an immune response. Antibodies produced as a result of the immune response can be used to detect the presence of a disease marker. Assays may also be configured to detect antigens. The antibodies or antigens are detected in specimens such as serum, plasma, urine and saliva.

Laboratories take into account a number of factors when choosing which type of device to introduce; for example, test performance, application, value in test algorithms, throughput, cost and training requirements.

Enzyme Immunoassays (EIAs)

EIAs are usually presented in a 96-well microplate as shown in the diagram opposite. The wells are coated with antibodies and/or antigens depending on the assay configuration. Each specimen or control to be tested is added to an allocated well.

After a wash step, antibodies labelled with enzyme, are added. The labelled antibodies react with a substrate that causes a colour change which can be measured with a spectrophotome- ter. The results can be qualitative and/or quantitative. EIAs are appropriate for screening large numbers of specimens.

Simple / Rapid Test Devices (S/RTDs)

These assays are available in several formats, but as their name suggests they are normally quick and easy to use. However, they are not suitable for processing large number of specimens. S/RTDs use antibodies in the same way as an EIA but the result is usually subjective. An example of a S/RTD is a lateral flow assay

.The test sample is added to one end of the device and the specimen passes up the membrane. If the specimen contains any antibodies/antigens that are specific to the test, then two bands will appear (reaction and control bands). If the specimen is negative then just the control band will appear.

Line immunoassays (LIAs)

LIAs have individual antigens applied as separate lines on a membrane strip. The strip is incubated with the test specimen followed by sequential addition of the test reagents. Any antibodies that bind to the antigens on the membrane will appear as a coloured line. This makes it possible to distinguish the specific antigens to which antibodies in a specimen are reacting and builds a profile of an individual’s immune response for a particular disease marker. Established criteria are employed to interpret patterns of reactivity. In the UK, LIAs are not used for initial screening of specimens, but as part of a supplementary or confirmatory test algorithm.

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Automated random access ‘closed’ analysers

These are usually large machines that automatically perform an assay in a closed environment. Once specimens are loaded on the system, the analyser will process the test through to completion.Automated analysers are useful for processing large numbers of specimens and they require less hands-on time.

The analysers often take up a large amount of laboratory floor space and only dedicated assays supplied by the manufacturer can be used.

Open Serology Processors

Open serology processors are bench top machines that automate EIA assays. Open processors can be independently programmed and there is no restriction on the choice of assay and reagents that can be placed on the system. This allows more flexibility than the ‘closed’ systems.

Agglutination Assays

Red blood cells or gelatin particles are coated with antigens. Antibodies in the specimen react with the antigen on the red blood cells, then agglutination will occur. If there are no antibodies present, then a small button will form

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