The quality assurance system in clinical chemistry.

The quality assurance system in clinical chemistry allows for the identification of errors and control actions to correct them. It is well known that laboratory errors can be classified as: pre-analytical, analytical and post-analytical. While pre-analytical and post-analytical errors are very difficult to identify, the analytical variability (both imprecision and inaccuracy) can be monitored with internal quality control (IQC) programs and external quality assessment (EQA) schemes. The purpose of IQC is mainly to verify the stability of laboratory estimates with time and therefore it is essentially a control of imprecision. IQC programs are based on the use of control samples which are analyzed in each analytical series. The easiest method of representing IQC data is by the use of Shewhart's chart, although "cusum" chart and Youden plot are often useful. As for the criteria according to which an analytical series should be accepted or rejected, the use of practical control rules is widely spread in laboratories. Participation in EQA schemes allows the laboratory to have a retrospective estimate of its performance in terms of both imprecision and inaccuracy, if definitive or reference methods are available. In lack of definitive or reference methods, consensus mean or median can be derived from the data obtained by all the participants or, in some cases, by the participants using the same analytical method (e.g. for analytes not yet completely characterized and measured with immunoassays.