Quantifying DNA loss using acid phosphatase screening for semen

In a criminal investigation DNA evidence is of high importance due to the nature where it isunique to an individual and therefore can be used for identifying a person of interest, a witness, victim and even to rule out the involvement of someone. The quantity of DNA on an item of evidence can often be quite minimal however and due to the processing of the sample to extract the DNA being a multi-step process it can mean that some DNA can be lost during this process. This makes it highly important to preserve as much DNA in a sample as possible by optimising these procedures. This research investigated the acid phosphatase (AP) test which is a presumptive test used by forensic scientists that detects the acid phosphatase enzyme which is found in high concentrations of semen. The test is used to give an indicator about whether semen may be present on an item of evidence. The procedure of the test involves the use of dampened filter paper which is applied to an evidence item and then sprayed with the AP reagent and will give a bright purple colour change if the result is positive. If a positive result is seen an area from the evidence item is swabbed or excised and sent for DNA profiling and quantification. Due to the nature of the test having many points of contact it means that there is a possibility that DNA is also being lost during the test. This research investigated where in the AP test loss may occur and how it can be minimised. The results showed that although DNA is being transferred to the filter paper the amount of DNA is quite small however full DNA profiles were able to be generated from the filter paper from some of the substrates tested. Interestingly it also showed that DNA transfer was occurring between the evidence item and the surface beneath it in this case a plastic sheet which when swabbed also produced full DNA profiles depending upon the substrate. The results also showed that it may be possible to directly test with the AP reagent onto the evidence item however more investigation into this is required.