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Discuss_the_factors_to_be_considered_before_selecting_a_commercially_available_diagnostic_assay._Illustrate_your_answer_by reference_to_specific_examples.

 

Numerous factors need to be taken into account when selecting a commercially available assay. The factors to be considered generally fall into 3 broad and interconnected categories;-

1. Actual Requirements

2. Practical considerations

3. Cost

Actual_Requirements

The perfect assay would be 100% sensitive and 100% specific, easy and rapid to perform, and inexpensive. No currently available commercial assay reaches that ideal and nor are any such assays likely to be available in the near future. Therefore, compromises would have to be made. The following are the sort of questions which need to be addressed:

- is the assay to be used as a screening test, or confirmatory test or both?

- what is the volume of workload expected for the test, and the turnaround time required?

- are there any other requirements eg. is the test to be used on an on-call basis, or is accurate quantification of the result required.

In general, the sensitivity of an assay is inversely related to its specificity. The virologist must decide whether it is more important to maximize sensitivity, specificity. It is desirable to maximize sensitivity in situations when the risk of a false-negative result outweighs the risk of a false-positive result. eg. the screening of blood donations for HIV, hepatitis B and hepatitis C. Maximum specificity is needed when the requirement is for a confirmatory assay eg. for newly diagnosed HIV infections, when it would be desirable to use assays such as Western blot or Liatek line immunoassay for confirmation. Maximum specificity is also required if the purpose is to identify negative patients eg. women negative for rubella antibodies during antenatal screening.

If the volume of workload is expected to be large, then it would be desirable for the assay to be as simple to perform as possible eg. in the case of antenatal screening for rubella antibodies, latex agglutination (eg. Rubelax), would be by far the most simple assay to use, in comparison with single radial haemolysis and ELISA (eg. Abbott rubazyme). Also, it may be opportune to consider assay systems which are readily automated. If the volume of workload is expected to be small eg. if the test is to be used as a confirmatory test for HIV antibodies, then it would not matter that much if the assay is time-consuming and tedious to perform eg. commercial Western blot (eg. Diagnostic biotechnology) and line ELISA (eg. Organon Liatek)

If it is desirable to have a result within the same day such as for on call purposes, or other reasons eg. if the local STD clinic wishes to have same day HIV results, then assays which require overnight incubation would be inappropriate. For on call purposes, the assay should be as simple and rapid to perform as possible. eg. latex agglutination (eg. CMV-scan) for CMV IgG, Abbott IMX MEIA for HBsAg, and Abbott rapid ELISA for HIV would be appropriate choices for pre-transplantation screening.

It may also be desirable to have some form of quantification eg. HBsAb titre following vaccination where accurate quantification is desirable. In such instances, EIAs will not give accurate quantification whereas RIAs would. However, chemoluminescence systems (eg. Kodak Amersham Amerlite) will be just as accurate as RIAs with mush reduced risk to the personnel.
 
 

Practical_Considerations

The following practical considerations ought to be considered: the format of the assay, the technical expertise required in order to perform it, and the reliability of the supplier.

Format of the assay - the main question to be asked is whether the laboratory is geared up to perform it. For example, has the laboratory got facilities to perform radioimmunoassays? If not, how practicable is it to set up such a facility?

Does the assay require specialized proprietary equipment in order to be carried out eg. in the case of EIA, most commercially available assays are based on the microtitre plate format and will thus only readily available washers and plate readers. However, many Abbott EIAs utilizes a bead system for its solid phase, and special washers and readers would need to be purchased. Similarly, although the Kodak Amerlite chemoluminescence uses a microtitre system, the whole set of proprietary washing, incubation and plate reading equipment would have to be purchased. In such a case, unless the volume of expected work is large, it would probably be uneconomical to purchase a complete system for just a single assay.

Technical expertise required - the level of expertise required to perform commercial vary from the exceedingly low, such as latex agglutination and highly automated assays such as Abbott IMX, to very high, such as HBV-DNA determination by in solution hybridization (Abbott), or the CMV antigenaemia test (Incstar Ltd.) The latter assays should only be carried out by senior technicians.

Reliability of the supplier

This is a very important factor when automated systems are considered for large volume work. Massive disruption will result should reagents run out and are not supplied promptly, and also should the equipment malfunction and repairs are not carried out promptly.

Cost

When considering the overall cost of a commercial assay, one should take into account the actual reagent cost, any overhead capital spending on equipment (including maintenance contracts), and separate labor costs. Obviously, if the volume of work is large, then it is important to keep the unit cost per test to be as low as possible. Current virus testing procedures are labor intensive. Although kits designed for automation usually have higher reagent cost, they are designed to be labor saving. Estimates of labor cost can be made by determining the amount of time required for the technician to perform the test. Suppliers of automated testing systems will often lend the equipment to the laboratory for a specified period free of charge for evaluation. However, they will often charge a higher price for the reagents than if the equipment were already purchased.

Assays specifically designed for confirmatory testing eg. Western blot and Liatek for HIV, or RIBA for hepatitis C are generally very expensive and often labor intensive. Assays to be used for on call testing should be as rapid to perform as possible as out of hours laboratory technician time is very expensive. Since in both cases, the volume of work is likely to be small, when one takes into account the reagent cost, one should take into account the reagents needed to set up the controls.
 
 

Evaluation_of_the_chosen_assay

Information about available commercial assays may be available from different sources. Comparison tests between different available commercial assays are routinely published by sources such as the Virus Reference Laboratory, the Federal Drug Administration of the U.S., and occasionally in various journals. Information about particular assays can also be obtained informally from colleagues in other laboratories already using the assay.

If the selected assay meets the requirements of the laboratory and has been validated by laboratories of comparable clinical experience, then little comparison testing is required. However, there may be two or more tests with similar claims and costs often are acceptable. The next step will be to conduct experiments to validate the claims and labor estimates for these tests. Where possible, the evaluated assays should be compared with recognized reference methods eg. EIA (eg. rotazyme) and latex agglutination assays for the detection of rotavirus antigen with electron microscopy, or tests for the detection of chlamydial antigens with chlamydial culture. When reference methods are not available as in the case of many serological tests, the assays should be compared to the one in current use in the laboratory and any discrepant results should be vigorously investigated: if it is necessary, be sent to reference laboratories for further investigation and the clinical picture of the patient assessed.
 

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