Cleaning a crime scene 2.0—what to do with the bloody knife after the crime?

The persistence of DNA on washed items as well as the DNA transfer has become a major subject of research in recent years, especially after the detectability of minor DNA traces was heavily increased by sensitive analysis methods. Nowadays, the attribution of a DNA trace to an individual is only rarely questioned, whereas the way of application of this DNA to an item is subject to much discussion and speculation. Additionally, the removal of DNA by cleaning or its possible persistence on an item despite a cleaning process are often important problems in court. The aim of this study was to investigate whether DNA traces (blood, saliva, epithelial cells) on different objects (knives, plates, glasses, and plastic lids) can persist on the surface despite cleaning by different methods like hand-washing or the use of a dishwasher. In total, 120 samples were collected from artificially constructed blood, saliva, and epithelial cell stains on objects with smooth surfaces after washing and analyzed by STR amplification. Samples taken after rinsing or hand-washing resulted mainly in complete DNA profiles (62.5% of samples), while cleaning in the dishwasher rendered almost everything completely DNA-free. Since in the hand-washing experiments a secondary transfer of DNA through the water could not be ruled out, additional transfer experiments were conducted with blood and saliva samples on plates. Here, a carryover of DNA traces could be demonstrated up to the fifth washed item.