Direct detection of DNA in complex biological samples bears several challenges regarding the selectivity and sensitivity of analyses. Therefore, DNA pre-extraction from bio-fluids is an emerging tool in biologically related fields. Specifically, a newly developing family of liquid biopsy techniques using PCR detection of circulating tumor DNA from blood serum or blood plasma could be significantly improved by harnessing fast and high-throughput DNA sample preparation. To address these needs, a 3D-printed device and a method based on gel electrophoresis combined with electrodialysis for the time-, cost- and labor-efficient preparative separation of DNA fragments from blood was developed. The proposed system also successfully eliminated large DNA fragments from the samples. Recovery for short DNA fragments was reaching up to 80 %. The method was tested on human genomic DNA and blood and blood serum spiked with DNA standards, and it significantly alleviated the signal of matrix DNA.