Dilute plasma biobanking protocol v1

This protocol presents methods for separation and biobanking of dilute plasma from human whole blood, specifically in parallel with a peripheral blood mononuclear cell (PBMC) isolation by density gradient centrifugation. Whole blood is diluted with Ca²⁺- and Mg²⁺ -free 1x PBS prior to Ficoll layering for PBMC isolation to increase the density differential of whole blood relative to separation medium to promote effective layering. After centrifugation in accordance with PBMC isolation protocol, dilute plasma available as supernatant is aliquoted into cryopreservation tubes for long-term storage at -80°C. Generated dilute plasma can be clarified prior to storage with additional centrifugation steps at 4°C or performed upon thawing, depending on study goals. Accurate quantification of analytes for proteomic assessment may be achieved by correcting through a dilution factor, considering (i) phlebotomy tube anticoagulant volume, (ii) total volume whole blood drawn, and (iii) added 1x PBS volume for PBMC processing.