Testing quality control samples is routine in the clinical laboratory but typically these are precision controls that monitor only the reproducibility (random error) of an assay and not trueness/bias (systematic error). To assess bias, accuracy controls that address both systematic error (trueness) and random error (precision) are needed. A properly prepared trueness control can be used to assess both bias and precision, as accuracy is defined by both random and systematic error. Providers of reference materials, such as metrology institutes and manufacturers are best suited to provide accuracy controls.