Analytical performances of SPAPLUS® turbidimeter for the quantification of albumin and IgG in serum and cerebrospinal fluid.

Abstract Objectives The intrathecal production of IgG is part of the diagnosis criteria for Multiple Sclerosis. Its assessment requires both quantitative (quantification of IgG and albumin in serum and cerebrospinal fluid (CSF)) and qualitative assays (isoelectric focusing). We have evaluated the analytical performances of the SPAPLUS® immunoturbidimeter (The Binding Site®) for the quantification of IgG and albumin in serum and CSF. Design and methods Within-day and between-day precision, linearity and carry-over were assessed. Results obtained with SPAPLUS® were compared to those obtained with the nephelometer IMMAGE® 800, including albumin quotient and CSF IgG index. Isoelectric focusing was performed and considered as the gold standard for assessment of intrathecal production of IgG. Results The within-day and the between-day precisions were obtained at two concentration levels and were below the recommendations of the manufacturer and of the French Society of Clinical Biology. Our evaluation confirmed the linearity of the assays and the absence of contamination. An agreement above 94% was observed between the results obtained with SPAPLUS® and those obtained with IMMAGE® 800. The use of the new reference material DA470k did not significantly modify IgG and albumin values. The confrontation of CSF IgG index and isoelectric focusing results led to a sensitivity of 79% and a specificity of 97% of CSF IgG index quantified on SPAPLUS® for the presence of oligoclonal bands at IEF. The sensitivity of intrathecal IgG calculated with the Reiber's hyperbolic formula was 47.4% and specificity was 97% for the presence of oligoclonal bands at IEF. Automatic rerun managed by the device for concentrations outside the measuring range was satisfactory. Conclusion The SPAPLUS® immunoturbidimeter displays good analytical performances for the parameters evaluated in this work.