MRD Application in Colorectal Cancer Patients

ctDNA is obtained from plasma rather than serum, as the latter contains more DNA released from immune cells during clotting. Blood should be drawn into a K2EDTA or cell stabilization tube (e.g., Streck cfDNA collection tube). Plasma isolation should be performed as soon as possible, no later than 24 hours, using K2EDTA (preferably within 4-6 hours) and, if using cell stabilization tubes, within 2-7 days, temporarily stored at 4°C. Sample disposal procedure: collect whole anticoagulated blood, gently invert to fully anticoagulate, plasma fractionation should be performed as soon as possible, no later than 24 hours, temporarily stored at 4°C using K2EDTA (preferably within 4-6 hours), and stored at -80°C for long term. The collected anticoagulant blood is placed in an ice box containing ice packs and transported vertically to a centrifugable laboratory. Centrifuge at 3000 rpm at 4°C for 15 min, take the upper layer, and aliquot 0.5 mL/tube into 1.5 mL centrifuge tubes. Cryopreserved at -80 °C, 1 mL of whole blood yields approximately 0.5 mL of plasma.