The concept of external quality assessment comprises different processes, which are used to do some type of evaluation of quality of results by the intervention of an organisation outside the laboratory.

The most common way of external quality assessment are those programs called programs of comparison between laboratories or programs for the external quality evaluation. These programs are commonly referred by the acronym EQAS (External Quality Assessment Scheme).

In the external quality control programs, many laboratories measure one or several components of the same control material without prior knowledge of values. The program organisation compiles the laboratories results and carries out a study that is later distributed to each laboratory informing them of the quality of their results.

Some days after having taken the measurements, the data arrive at the laboratory, that is why they are not useful for daily control.

External control can not substitute internal quality control, but it complements it due to the capacity for detecting errors in a measurement procedure under stable conditions. Internal control only detects deviations in stable behaviour. External control is mainly used to identify systematic error, although it can also be used to reinforce the control of random error.

The international external quality assessment program organised by BioSystems S.A. (PREVECAL) offers laboratories the possibility to complete their internal control scheme with an objective estimation of the quality of their measurement procedures.