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

Participants (230) from Germany and 20 laboratories in 11 European countries took part in a newly designed cerebrospinal fluid (CSF) survey distributed by INSTAND. Conventional proficiency testing for albumin, IgG, IgA, and IgM in CSF and serum, for total protein in CSF, and for oligoclonal IgG 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 immunoglobulin 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 accreditation bodies. The sensitivity of the methods for quantifying IgA and IgM in CSF and for detecting oligoclonal IgG fractions is discussed.