The missing piece in your jigsaw

1st April 2013

The quality assurance of analytical systems is important to ensure the accuracy and precision of test results. Analysers are calibrated using serum-based samples to mimic the matrix of the patient and may only be required once every few days. However, to ensure accuracy of the detection on a daily basis, quality control serum is used to check the analyser is still accurately calibrated and the results are reliable.

Quality control sera contains the required levels of an analyte to validate each test result derived from the analyser, over its clinically significant ranges. Multi-sera is simply serum that contains a mix of different analytes being tested on the analyser. This makes quality control much quicker, simpler and more cost-effective when running a number of tests.

The choice of supplier can have a massive influence on the ease of use and suitability of quality control sera. As with all essential pieces of laboratory equipment, decisions on the most appropriate sera and EQA schemes should not be taken lightly.

Quality control is defined as a process to identify errors in a procedure. This is a reactive process, where errors have already occurred. Without quality control, errors in a clinical setting can lead to patient misdiagnosis, delays in treatment or administration of the wrong treatment and increased costs associated with retesting.

How can analytical quality and performance best be controlled in a laboratory setting and how can you be sure that your results are within acceptable parameters for optimal patient care?

Internal quality control

Internal quality control involves a set of procedures that daily monitor analytical processes and their emergent results. For the best assessment, third party quality control sera are essential. This has an immediate effect, so should be able to control current processes, as opposed to merely examining outputs over a set time. Both precision and accuracy can be assessed using QC sera and steps can be taken immediately, to rectify any test bias or analytical error. Using QC sera, results within 1 or 2 SDs of the target value indicates that the analytical system is running well and test results can be relied upon.

Finding the best fit for your internal quality control (IQC) should not be confusing, as everybody wants the best possible sera. Two options are available for IQC; unassayed and assayed. The first is often a cheaper option, but has the downside of not being supplied with target values, thus the laboratory has to generate a minimum of 20 sets of data and perform calculations to assign target values and acceptance ranges. Another way for the customer to assign target values is from a peer group programme, however, the target values obtained are only as good as the inputted data.

To determine the accuracy of the test, assayed controls are required. These sera are supplied with target values and an acceptable range for each analyte within the sera. To cover the full clinical range, it is better to run three levels of sera for each analyte under test, to ensure the test is suitable across the whole range and not the "normal" range, as may be the case if pooled sera was used. The best quality control manufacturers will also supply reference method values and traceability to international reference methods, so you can be sure the target values quoted are correct. This gives additional accuracy and ensures confidence in the test results.

Additional advantages of using assayed controls include the elimination of cost and time required to generate target values and ranges for each analyte. Randox Laboratories Ltd, for example, use a database of more than 6000 laboratories to set their ranges. This size of test community allows the sub-division of values to be assigned for specific methods and instruments. The material is also traceable to reference methods and reference materials, so improving accuracy and confidence further.

The next aspect of good IQC is the frequency of testing. As a rule of thumb, testing should occur at a frequency sufficient to detect an error in the process when it occurs. The frequency of running controls thus depends on the frequency of the tests, control stability, the numbers of samples processed and the method used. It is advised that materials should be assessed every day, at a frequency dependant on the reproducibility of the test and at concentrations that challenge the reportable range.

A good quality control assay should be able to test the instrument, the reagents and the operator. This is where third party controls make the difference.

In many laboratories, pooled serum is used as an independent control, but this is not independent or controlled. Pooled serum is likely to be within the normal range, so will not evaluate the test system across the clinically significant range. Stability of the serum will not have been evaluated and there will be no long-term supply, so the ranges within your controls may change. This has the capacity to bias your test as each new batch of pooled sample is introduced. If the patient samples drift out of range, is this due to the analyser needing re-calibrated, the test reagents going off or because your pooled control sample is going off? This extra level of complexity is avoidable, if control sera are used from a reputable supplier.

Control sera from a reliable manufacturer has not been designed or optimised for one test or system, therefore is similar to real patient samples. They also perform independently of reagent batch changes and provide an unbiased assessment of all aspects of the procedure. A big advantage of third party controls is that they can offer consolidation, ie more than one target can be assessed using one serum sample. This reduces the need for purchase and storage of multiple controls, leads to fewer invoices and the management of inter-lab peer group programmes becomes much easier.

Peer group reporting

To move your IQC to another level, you may want to consider peer-group reporting. This allows inter-lab reporting, using the same test and instrumentation. Peer-group reporting is not an external quality assessment, as the same controls are tested daily, and the levels are pre-determined. In the majority of cases, error bias can be minimised quickly and increasing the number of participating laboratories gives greater confidence in the consensus mean value. Clearly, to run a successful peer-group IQC, it is necessary to have a scheme that can provide updates 24/7 on the performance of you lab against your peers. In this respect, immediate remedial action can be taken to re-calibrate the instrument, without long-term reporting errors.

External quality assessment

The final and most complete element in the quality control spectrum is External Quality Assessment (EQA). This has been recognised as an essential part of overall quality control management in many countries, to the extent that many national EQA schemes are compulsory, for example CAP in the US. To be an accredited laboratory, you need to run an international EQA scheme. A compulsory national scheme may not be international, so laboratories will also need to participate in an international EQA scheme, if they wish to be an accredited laboratory. EQA provides a means of assessing the analytical performance of a laboratory compared to others using the same instruments and methods. An individual laboratory is assessed using blind samples that are analysed as if they are patient samples. Results are sent to the co-ordinating programme provider for interpretation, after which a report is sent back to the participating laboratory. This maintains and improves the analytical quality of all of the laboratories participating in the scheme, thus raising standards worldwide, which has to be good for patients, who are, after all, the end users of the tests under scrutiny. If one analyser is underperforming, this can be identified, as can poor staff performance. In another aspect, should this be required, comparisons can be made between different analytical methods and instrumentation. Such comparisons would not normally be available in IQC schemes.

To be worth consideration, an ideal EQA scheme should have sufficient participants enrolled in the particular programme you wish to be involved in and there should be effective consolidation of programmes. To avoid a fridge full of controls, consolidation of control sera is a valuable factor for a busy laboratory. If a clinical chemistry analyser, for example, runs 63 different analytes in one laboratory, the best fit would be control sera that covers all these in a single vial. This can provide considerable time and cost savings.

An effective EQA programme must report frequently enough to allow for timely intervention of problems identified and the material must be recognised as being of the highest quality, preferably through recognition from international accreditation bodies.

Now that the basics of the quality assessment have been covered, the fine tuning to suit you particular needs comes into play. Important considerations for optimising your confidence may seem obvious, but are worthy of mention again. The primary function of controls is to set parameters for patient samples, so the best controls will be the same matrix as the sample under test. This is not always the case, however, as, for example, some human controls on the market have animal proteins added for cost savings, which can cause matrix effects, with antibody based tests.

This can result in differences being observed between different batches of reagents which can cause confusion with the customer as they have to decide is it a problem with the control or the reagent. It also follows that for veterinary applications, animal controls would be appropriate.

The choice of liquid or lyophilised control is, in many cases, a personal one, but big factors are ease of use, shelf life and consistency. Liquid stable controls have the advantage of being ready to go, but may suffer loss of shelf life, whereas lyophilised samples have a longer shelf life, but require careful reconstitution, to ensure the correct values are attained.

In summary, internal QC, peer group and EQA programmes ensure quality results from whatever analysis you perform. This applies to clinical use or clinical trials, but in all cases, having confidence in results makes sense. Finding the right scheme will be determined by the size, duration and complexity of each trial, but for every test, you can be sure there will be a suitable control that will fit. The clinical trial jigsaw will then be complete.

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Dr Martin Crockard is with Randox Laboratories Ltd, Crumlin, Co Antrim, UK.





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