Methodological and statistical issues in developing an External Quality Assessment scheme in laboratory medicine:...

External Quality Assessment (EQA) schemes are well-established tools with which to evaluate, monitor and improve the output quality of clinical laboratories, recognising that high quality laboratory medicine is essential for patient care. EQA programs involve the testing of multiple laboratories and the statistical comparison of their results, according to a multistep workflow. New clinical laboratory activities, such as biomarker research, require new EQA schemes. Critical elements in designing EQA programs are choosing the statistical methods and defining reference values and control limits. This article summarizes the key features of an EQA scheme, including designing the study, identifying reference values and control limits for qualitative and quantitative data, and graphically reporting laboratory performance statistics. These steps are illustrated with examples taken from the authors' experience in national and international quality assessment schemes for biomarker research.