Analysis of protein S-100B in serum: A methodological study

Background: Dysfunction and damage of the human central nervous system can be detected with biochemical markers, and protein S-100B is the best-established such marker. The aim of this study was to evaluate whether the protein is stable during long-term storage, to establish reference values for the new Elecsys® S100 test and to compare this new method with the Liaison® Sangtec® 100 test. Methods: We analysed blood samples from 118 blood donors and 196 patients with subarachnoid haemorrhage or head injury. The long-term stability of S-100B in frozen serum samples was evaluated with repeated analysis in 1997 and 2003 using an immunoradiometric assay. Method comparison between the Liaison® Sangtec® 100 and Elecsys® S100 tests was performed using Bland-Altman difference plots. Results: Serum concentrations increased significantly during long-term storage (mean difference 0.15 μg/L; ±2 SD, 0.55 μg/L). Serum measurements using the Elecsys® S100 method in 118 healthy blood donors showed S-100B levels between 0.02 and 0.08 μg/L (mean 0.05). The 95th percentile was 0.07 μg/L. The Liaison® Sangtec® 100 test usually measured higher concentrations than the Elecsys® S100 method, and the difference between the two methods increased with increasing concentrations. The mean difference between the methods was 0.14 μg/L (±2 SD, 0.39 μg/ L). Conclusions: Protein S-100B is not stable during long-term storage and the two analytical methods are not interchangeable.