Accuracy of cerebrospinal fluid Aβ1-42 measurements: evaluation of pre-analytical factors using a novel Elecsys immunosassay.

BACKGROUND: A decreased level of Aβ1-42 in cerebrospinal fluid (CSF) is characteristic of Alzheimer disease and often used to support clinical diagnosis. The measured concentration of CSF Aβ1-42, however, depends strongly on several pre-analytical and analytical "confounding" factors such as sample collection, material of testing tube, CSF handling and storage procedures (e.g. transfer to new tubes after centrifugation, freeze-thaw effects). As a consequence, substantial variations in the measured levels of this biomarker are observed even for the same sample. This study investigates whether the accuracy of quantitative analysis of CSF Aβ1-42 can be improved by pre-analytical treatment of CSF with agents that could potentially reduce a freeze-thaw and adhesion-related depletion of Aβ1-42 from CSF, including modulators of Aβ aggregation and cryoprotecting or anti-adhesion agents.

METHODS: The concentration of CSF Aβ1-42 was assessed with a novel Elecsys immunoassay developed for quantification of Aβ1-42 in human CSF.

RESULTS: Low-molecular weight Aβ oligomerization inhibitors, β-sheet breaker peptides, or the mid domain 4G8 antibody do not improve the stability of CSF Aβ1-42 during a repeated freeze-thaw treatment. Cryoprotecting agents reduce a freeze-thaw dependent loss of Aβ1-42 only when spiked to CSF to final concentration of 300 mM or higher. Adhesion of Aβ1-42 can be prevented by pre-treating CSF with Tween or by using tubes with a siliconized surface.

CONCLUSIONS: Between-center variability in measured level of CSF Aβ1-42 can be reduced only by standardized CSF collection into one specific tube that, without centrifugation, transfer or other types of pre-analytical processing, is directly analyzed after sample collection.