A novel three-step DNA extraction method for mixed bloodstains

Mixed DNA samples from at least two contributors can be present at a crime scene, which could be the most crucial piece of genetic evidence. The mixed stains in sexual assault cases are typically separated using differential lysis procedures (a two-step method). Blood mixed stains, however, are usually difficult to separate. In this work, we propose that a mixed stain comprises three layers, that is, (1) the upper layer which is primarily made up of cells from one contributor; (2) the middle layer which is a similar mixture from two contributors; and (3) the lower layer which primarily comprises cells from the other contributor. Based on this concept, a novel three-step DNA extraction method was proposed to solve the challenge involving bloodstains from two contributors. In the experiment, we extracted three layers DNA from mixed bloodstains using three steps. As a result, single-source DNA and approximate single-source DNA were detected from steps 1 and 3, respectively. This study demonstrates that the DNA from some mixed blood stains could be effectively separated following an appropriate extraction strategy, providing valuable insights, and serving as a reference for future examination of blood mixtures.