SpermFACS: Validation of a Highly Sensitive Sperm Cell Sorting Method for Sexual Assault Casework Analysis

Area of Science:

• Forensic Science
• Molecular Biology
• Genetics

Background:

• Sperm cell identification is crucial in sexual offense investigations.
• Differential lysis (DL) is the standard but often inadequate method for sperm separation.
• Limitations of DL include time consumption and poor DNA profiles from low sperm-to-epithelial cell ratios.

Purpose of the Study:

• To establish and validate a fluorescence-activated cell sorting (FACS) method, termed spermFACS, for routine sperm cell separation from vaginal swabs.
• To meet ISO/IEC 17025 guidelines for forensic implementation.
• To assess spermFACS's effectiveness in challenging forensic samples.

Main Methods:

• Development and validation of a FACS-based methodology (spermFACS).
• Separation of sperm cells from vaginal swab samples.
• Comparative analysis of spermFACS against differential lysis (DL).
• Assessment of DNA profile quality from mixed cell populations and aged samples.

Main Results:

• spermFACS successfully recovered informative male autosomal DNA profiles from mixtures with sperm/epithelial cell ratios as low as 1:7500.
• Effective DNA profiling was achieved from postcoital samples collected up to 120 hours after intercourse.
• spermFACS demonstrated superior separation efficiency and purity compared to DL, supported by likelihood ratio calculations.

Conclusions:

• spermFACS is a valuable tool for forensic analysis, particularly in delayed-report sexual assault cases.
• The method shows potential for deconvoluting complex mixtures, such as in multiple offender rapes.
• Integrating spermFACS into forensic workflows can enhance offender identification and case resolution.