Additional fixation improves the performance of SPERM HY-LITER Express™

Abstract The identification of sperm cells is important in forensic casework as it may confirm sexual activity in cases of alleged sexual assaults. In addition, the presence or absence of sperm cells is important for deciding on the analytical strategy for STR-typing of such samples. Therefore, using a sensitive and robust method for detection of sperm cells is important for forensic casework. During internal validation of SPERM HY-LITER™ Express, we experienced that the fixation of cells to the slides may represent a challenging step in the protocol. Cells that are not properly fixed to the slides may be lost during the subsequent staining and washing steps. To address this challenge we evaluated the effect of introducing an additional 2min fixation step to the standard protocol. The modified protocol was tested on slides from Independent Forensic and Menzel Glaser. For mock samples prepared by mixing sperm cells and vaginal cells, significantly more sperm cells were identified for both slide types when samples were processed with an additional fixation step compared to the standard protocol. Furthermore, the examination of a dilution series of sperm cells indicated that also the sensitivity of SPERM HY-LITER™ Express improved by modifying the fixation. For post-coital samples, our preliminary results demonstrate improved results with the modified protocol for some, but not all of the samples. Although the sample set was limited and further testing is required, our results so far indicate that the modification may improve the performance of the kit.