Investigating the age of blood traces: how close are we to finding the holy grail of forensic science?

Blood traces found at crime scenes often comprise pivotal information regarding the events and individuals associated with the crime. Nowadays, even minute amounts of blood allow retrieval of a whole host of such ‘profiling’ information: e.g. diet, life style, age, gender. However, establishing any forensic value of such traces necessitates a veritable connection to a crime. The age of a blood trace, i.e. the time of its deposition, is crucial in this effort. This far-reaching forensic implication as well as the lack of currently validated and accepted trace dating methods, render blood stain age estimation the holy grail of forensic science. In its pursuit, several methods which determine the time since deposition of blood traces by probing different aspects of the trace degradation process have been proposed and explored. The present chapter collates and discusses current research investigating some of these blood trace ageing methods and their practical application in three categories. The first category comprises techniques which require trace sampling and consume these samples in their entirety during the analysis process. Similarly, the techniques in the second category require sampling of the blood trace but leave the sample intact for further analysis. Lastly, the third group of methods requires neither sampling nor contact. This, in turn, allows in situ analysis of the trace in question. The following operational aspects pertaining to these three categories are discussed in more detail: (i) required sample preparation, (ii) practical implementation and (iii) necessary operational skills. These aspects largely determine the suitability for forensic practice. Technology maturity (i.e. practical applicability) is quantified using the Technology Readiness Levels (TRL) as defined by the NASA/Airspace systems.