Metrological traceability and clinical traceability of laboratory results - the role of commutability in External Quality Assurance.

The role of an External Quality Assurance (EQA) program is generally seen as providing a service to routine laboratories that their analytical performance is satisfactory and stimulating corrective action in the event of poor results. It is recognised that an ideal EQA program uses materials that are commutable with patient samples and have values assigned by higher-order reference methods. Despite this, most routine EQA programs use materials without verified commutability and use consensus means (based on either peer group or all laboratories) as target values. We propose an ongoing role for EQA programs using non-commutable materials and consensus targets to support the measurement services of routine laboratories. This is provided the relevant comparators supplied by the laboratory, e.g. reference intervals and clinical decision points, are based on the same or equivalent measurement system as is used by the laboratory. Materials without verified commutability often have certain practical advantages, which may include the range of analyte concentrations, verified stability, replicate samples and, significantly, lower costs. Laboratories using such programs need to be aware of the limitations, especially comparing results from different measurement systems. However, we also recognise that as well as individual laboratories, data from EQA programs informs manufacturers, professional organisations, clinical guideline writers and other medical bodies For consideration beyond an individual laboratory, proper assessment of differences between measurement systems (results harmonization) and demonstration of correct implementation of metrological traceability (methods trueness) become vital, and for that purpose, commutability of EQA materials and traceability of target values are required.