Evaluating the efficacy of DNA differential extraction methods for sexual assault evidence

•A semi-quantitative analysis of the differential extraction procedure is presented.•The effectiveness of the differential extraction is described by a ratio termed the “SPRED”.•The final ratio of sperm DNA to non-sperm DNA in the sperm fraction can be predicted by the SPRED and the ratio of sperm DNA and non-sperm DNA on the swab.

Analysis of sexual assault evidence, often a mixture of spermatozoa and victim epithelial cells, represents a significant portion of a forensic DNA laboratory's case load. Successful genotyping of sperm DNA from these mixed cell samples, particularly with low amounts of sperm, depends on maximizing sperm DNA recovery and minimizing non-sperm DNA carryover. For evaluating the efficacy of the differential extraction, we present a method which uses a Separation Potential Ratio (SPRED) to consider both sperm DNA recovery and non-sperm DNA removal as variables for determining separation efficiency. In addition, we describe how the ratio of male-to-female DNA in the sperm fraction may be estimated by using the SPRED of the differential extraction method in conjunction with the estimated ratio of male-to-female DNA initially present on the mixed swab. This approach may be useful for evaluating or modifying differential extraction methods, as we demonstrate by comparing experimental results obtained from the traditional differential extraction and the Erase Sperm Isolation Kit (PTC©) procedures.