Acid phosphatase test on Phadebas^® sheets - An optimized method for presumptive saliva and semen detection

The precise and efficient detection of semen and saliva in sexual assault case-work items is a critical step in the forensic pipeline. The outcome of this stage may have a profound impact on identifying perpetrators as well as on the investigation process and the final outcome in court. Semen detection is usually based on the activity of acid phosphatase (AP), an enzyme found in high concentration in the seminal plasma. Amylase, an enzyme catalyzing starch hydrolysis is found in high concentrations in saliva and therefore is a useful target for its detection. To screen case-work items, both presumptive tests require transfer of biological material from the item to paper in a moisturized environment. Since semen and saliva may appear in the same item, it is required in some cases to perform the tests one after the other. This may reduce the chances of identifying all stains on the item and obtaining a DNA profile. In the present study, we applied the AP biochemical test on a Phadebas^® sheet, a commercial starch containing paper used to detect saliva. This approach was found to be sensitive enough to detect diluted semen (1:50) after performing the Phadebas^® press test. In addition, it enabled detection of adjacent saliva and semen stains and stains containing a semen-saliva mixture. Finally, a DNA profile was successfully obtained from the Phadebas^® sheets after semen detection, a useful feature if the original item is lost or damaged. Taken together, this method provides a practical, reliable and convenient tool for screening sexual assault items of evidence.