[External quality control in the medical laboratory: evaluation of 12 ring trials 1994-1996].

The regulations for reimbursement of laboratory tests provide that such tests will be paid for by social insurance institutions only if the laboratories participate in internal and external quality control schemes. Twelve surveys of the external quality assessment scheme of MQ Zürich (Association for Medical Quality Control) were evaluated. We analyzed the imprecision and inaccuracy of the participant results depending on the analytical system or methods used. Furthermore, the number of participants who met the quality criteria published by FKGRAL (Expert Committee for Overall Revision of Analysis List) was determined. The deviations from the internationally recommended or reference methods respectively were within +/-33% for the metabolites and enzymes if native plasma was used as control sample. If lyophilized samples were used, 5 deviations observed were > +/-33% (maximum +99%). For hematologic parameters the deviations were in the range of +/-10%. The CV's were 5.7-17.6% for wet chemistry methods used by the participants and 4.5-15.1% for the other methods. (Cobas Ready, Ektachem, Reflotron, Vision). For hematologic parameters we found CV's between 4.5 and 14.0%. 69-83% of the participants using wet chemistry methods met the FKGRAL criteria, while 86-98% of participants using one of the other system obtained adequate results. The corresponding figure for the hematologic parameters was 83-93%. The nature of the control samples (native samples or lyophilizate) did not influence the number of participants who successfully passed the survey. The study showed that the surveys are an adequate tool for determining participants with inadequate analytical performance, and in many cases the survey results make it possible to propose the necessary educative measures.