Analytical performance specifications for EQA schemes - need for harmonisation.

External Quality Assurance (EQA) is a vital tool in laboratory medicine to assess individual laboratory analytical performance and also the differences between the results from different laboratories. This information is also useful for professional bodies and manufacturers as part of post-market surveillance. The process involves the measurement of one or more samples by many laboratories and then assessment of the results. Individual results are generally assessed by how far they lie from a target, which may be established using reference methods or a median of some or all of the submitted results. The distance of a result from the target is compared with analytical performance specifications in order to assess the analytical quality. One of the uses of the Stockholm hierarchy of performance goals is to set the performance specifications for analysis of EQA results. Fifteen years after the Stockholm consensus meeting, EQA analytical performance specifications appear to still vary widely between EQA providers. This can be due to a range of factors, including the rationale for setting the criteria, the expected response to a failure to meet the specified performance, the clinical meaning behind meeting the specifications, and the possible need for further analytical improvements. There are also differences in the models chosen to set the criteria, usually either state of the art or biological variation, and then differences in how these are applied. While harmonisation of EQA performance specifications may be some time off, all EQA providers should define the nature of their specifications and the basis for their selection and make this information available to customers.