Non-differential DNA extraction of postcoital samples submitted as evidence for investigating sexual assault cases in the Philippines

he number of reported sexual assault cases continues to rise yearly in the Philippines. One major step towards addressing this problem is to use DNA test results in rapidly and accurately identifying assailants. The common practice among forensic laboratories overseas is to perform differential DNA extraction to isolate sperm fraction. For this study, we employed non-differential methods to maximize DNA recovery. We evaluated two nondifferential DNA extraction procedures namely a phenolchloroform-isoamyl alcohol (PCI) method and a silica-based method for handling post-coital samples. The procedures were assessed based on total human DNA yield, quality of DNA extracted from single-source semen samples, and the proportion of male component from post-coital vaginal swabs and stains. Higher DNA yields but lower DNA quality resulted from the PCI method. The silica-based method using QIAamp DNA Micro kit produced better quality DNA, resulted in higher male proportion for male-female samples, was faster and less tedious, minimized exposure to hazardous chemicals, and generated less waste. In the Philippines, where biological samples may be compromised due to the tropical climate favoring enzymecatalyzed degradation of DNA, as well as delays in a victim reporting the incident, we recommend a non-differential silicabased method to maximize recovery of good quality DNA. This should be followed by statistical interpretation using a continuous model to objectively evaluate results that contain the DNA profiles of victim and assailant.