2.3. cfDNA Extraction

2 mL of plasma was used for the cfDNA extraction using the IDXtract kit (ID-Solutions, Grabels, France), a silica membrane nucleic acid extraction system, according to the manufacturer’s instructions. The efficiency of extraction process was assessed by adding an exogenous internal extraction control (ICE) in every plasma sample prior to extraction and by using a negative and a target positive control extracted in the same way as plasma samples. The final elution volume of extracted cfDNA was 65 μL. In parallel, another identical aliquot of 2 mL plasma for each sample was extracted using the cobas^® cfDNA Sample Preparation Kit (Roche Molecular Systems, Inc., Pleasanton, CA, USA), a system that uses columns with a glass fiber filter insert, according to manufacturer’s instructions, to a final volume of 100 μL of elution buffer.