There is a risk to patients of misdiagnosis or incorrect treatment because of bias between results reported from different laboratories/methods for the same measurand. This risk can be identified and hopefully reduced by using External Quality Assurance/Proficiency Testing (EQA/PT) programs when commutable samples are used to identify these differences between laboratories using the same or different methods for the same analyte. Differences may arise due to the traceability of the calibrators, assay imprecision, differences in analytical specificity, and individual laboratory practices.