External quality assessment in clinical neurochemistry: survey of analysis for cerebrospinal fluid (CSF) proteins based on CSF/serum quotients. Clin Chem

11 European countries took part in a newly designed cerebrospinal fluid (CSF) survey distributed by INSTAND. Conventional proficiency testing for albumin, lgG, IgA, and 1gM in CSF and serum, for total protein in CSF, and for oligoclonal lgG in CSF and serum was combined with evaluation and interpretation of CSF/serum quotients in quotient diagrams. The correct detection of a blood-CSF barrier dysfunction and the pattern of intrathecal immu-noglobulin synthesis was judged. The accuracy of CSF/ serum quotients and their clinically relevant interpretation was given first priority as a new concept in quality assessment. The main result of the surveys was to confirm that CSF/serum quotients of proteins represent method-independent values approaching the quality of reference values. This finding has consequences for internal quality control of CSF analysis and for accredi-tation bodies. The sensitivity of the methods for quanti-fying IgA and 1gM in CSF and for detecting oligoclonal IgG fractions is discussed. Indexing Terms afbumu/immurog!obullns/o1igodonaI IgG/inLema/ qu assessment/proficiency testing This report is based on the Harmonized Proficiency Testing Protocol of the International Standardizing Organizations (1); relevant terms from this protocol are defined in the Appendix. Quality assurance in medical laboratories, as practiced in many countries (2), involves external quality assessment (EQA) and internal quality control.1 External assessments usually involve interlaboratory surveys organized by an inde-pendent EQA organizer. Here I present experience with a newly designed sur-vey for cerebrospinal fluid (CSF) analytes distributed twice a year (1990-1994) by the Institut f#{252}rStandardis-ierung und Dokuinentation (INSTAND), Dusseldor