Specimen materials, target values and commutability for external quality assessment (proficiency testing) schemes.

BACKGROUND The objective of external quality assessment is to evaluate clinical acceptability of laboratory results. It is desirable to evaluate intermethod harmonization and traceability to a reference system. METHODS Conventional matrix-modified processed materials are used in most programs, because they can be produced in large quantities, can have multiple abnormal analytes in the same vial and have excellent stability. The principal limitation of these materials is non-commutability which makes them unsuitable for traceability or harmonization evaluation. Peer group evaluation is used which allows an individual laboratory to confirm appropriate use of a measurement technology and a manufacturer to monitor uniformity in their calibration transfer process. Authentic clinical specimen pools provide commutability but are limited by the quantity available and number of analytes at pathologic levels in the same vial. Hybrid approaches have used authentic clinical specimen pools in conjunction with non-commutable processed materials to determine method-specific matrix-corrected target values which have enabled evaluation of traceability to reference methods. CONCLUSIONS Conventional processed materials are limited by non-commutability. Pooled clinical specimens are limited by availability. When used together, external quality assessment can evaluate traceability to reference systems and harmonization between test procedures.