THE INFLUENCE OF MATRIX TYPE, DIURNAL RHYTHM AND SAMPLE COLLECTION AND PROCESSING ON THE MEASUREMENT OF PLASMA ß-AMYLOID ISOFORMS USING THE INNO-BIA PLASMA Aß FORMS MULTIPLEX ASSAY

Objective: The aim of the study was to determine the extent to which plasma matrix types, diurnal rhythm and sample collection and processing procedures contribute to overall variability of measurements with the INNO-BIA plasma A forms assay. Methods: Plasma samples from healthy volunteers were collected at BARC-CRI. Analyte concentrations from various plasma matrix types (EDTA, heparin, fluoride) were compared to serum after collection of blood in commercial plastic and glass tubes. Sample processing variables including time and temperature before and after centrifugation, centrifugal force and plasma dilution factor were also investigated. Diurnal variability in plasma A isoforms was determined in 29 healthy volunteers by analysis of EDTA plasma specimens serially collected over 24 hours and stored frozen following oral administration of a placebo treatment. All plasma samples from a given individual and experiment were analyzed in a single analytical run. Results: Highest A levels were obtained using EDTA-plasma samples (in contrast to serum, heparin, citrate, or fluoride). Addition of aprotinin to EDTA plasma had no effect on A peptide recovery. The elapsed time and temperature exposure, before and after sample processing affects the recovery of A isoforms. Analyte recovery was not significantly affected by the presence of platelets in plasma samples. At the subject level, analysis of serially collected EDTA plasma specimens from healthy volunteers revealed no evidence of diurnal variation in any of the A isoforms investigated and results from samples collected on a monthly basis showed only very limited intra-individual variation. Conclusions: Optimal recovery of A peptides was obtained from blood drawn into EDTA tubes and processed within 4 h. Plasma that was refrigerated after separation and analysed within 4 h gave comparable results to samples immediately processed and frozen at -70 °C.