Laboratories can often check the reliability of their results via proficiency testing. In addition to this, they can also use reference materials: materials for which the composition is known. A laboratory then, for example, purchases a pot of soil, a vial of blood or a gas mixture with a known composition. The laboratory results are compared against this material. If deviations are detected it will be investigated in which way the laboratory can improve its test method.
Producers of reference materials supply the materials that allow laboratories to conduct checks. The RvA accredits producers of reference materials on the basis of the ISO 17034 standard.
You can use the search engine or consult the list of accredited bodies to find details of all producers of reference materials accredited by the RvA. Policy document BR010 describes the various fields of work relating to testing.