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COMPARABILITY ASSESSMENT


indispensable data for the most important part of the analytical phase of method comparability studies – measurement uncertainty estimation.


Measurement uncertainty Measurement uncertainty (MU) must be used to compare analytical methods. It is impossible to compare methods, and to validate them as being appropriate, if we do not know what the expected measurement uncertainty is for each. Measurement uncertainty bridges analytic performance and clinical interpretation. All relevant uncertainty contributors are considered for inclusion into the preliminary MU budget for the assay under test. Data for the index method should already be on file. The important thing to note is that it does not involve any additional testing or experiments during the study. It is just applying the data in a way that means they can be applied directly to the comparability. Examples are assessment of bias, calibrators and traceability of results.


Bias assessment


Bias assessment comes in two forms. First there is identification of bias between our two methods under test – this assumes the index method as the ‘true’ value. Second is assessment of bias


in either method, individually, from the ‘true’ value. We already know we cannot ever determine the true value but we can determine whether we are biased against international standards. Our index method will have been assessed regularly for bias using consensus values applied by external quality assessment (EQA) schemes. Bias should be corrected if possible, using an international standard to confirm the extent of the bias, and the uncertainty of that correction incorporated into the MU.


Calibrators


Calibrators, if used, and their associated uncertainty, are intrinsic to the assessment of measurement uncertainty – and the impact may be different depending on the method. It is essential that this is fully documented and, if different calibrators are used for each method, a comparison of the traceability chains, and of their performance for each methodology, should be explicit to ensure that the results are interchangeable.


Traceability of results


In the absence of the gold-standard methodology (which should be traceable through a chain to the SI system of units), traceability of the assay under test is limited to the index method. Reports


should explicitly outline at the outset what metrics are used and how they are interpreted with respect to their traceability chain. Armed with the data from our


measurement uncertainty evaluations for both methods, we can be confident in knowing that if we observe a difference above the threshold we have set, we can attribute that to a variance other than as a consequence of MU (ie a genuine analytical difference between methods). That being said, the implication in the clinical setting is yet to be established, and the clinical comparison follows.


Clinical performance Sensitivity and specificity Sensitivity and specificity are always quoted. They are a useful starting point for our diagnostic assessment. We will cover their derivation in the worked examples in the coming articles in this series. Importantly, we must accept that we have not verified these data, which is the point of the comparison study. We accept that they are an essential part of the initial assessment, for a proof of concept. Without that, the assay would not be available on the market for us to use. The limitations of these data are important. Often, clinical studies may be quoted, but, even though we consider


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