The evaluation of tests for the identification of semen

Master Thesis


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University of Cape Town

The identification of bodily fluids for forensic purposes is typically classified as either presumptive or confirmatory. Presumptive tests (PT) are conducted first to screen for certain compounds which are relatively specific to particular fluids. Confirmatory tests are used to confirm the identity of a body fluid. Semen is one of the most common bodily fluids encountered in sexual assault cases and contains high concentrations of the acid phosphatase (AP) enzyme. The brentamine FB reagent reacts with the AP that is present in semen, and turns purple. If the colour change is observed within a specific time threshold, it is considered presumptively positive for semen. Cut-off time varies considerably between forensic laboratories, but in South Africa, the cut-off time is defined as 65 seconds. Additionally, semen may be considered to be from human origin if it reacts within 50 seconds. These cut off times have been arbitrarily defined, and there is little research in a local context to substantiate or inform the threshold time for the brentamine FB test for semen. Therefore this study assessed the sensitivity, specificity and kinetics of the brentamine FB test on semen from South African male volunteers (n=15), canines (n=2) and various fruit extracts and compared these results to purified human AP. Each semen sample was subjected to the PT in an indirect and direct method, and these tests were performed both on fresh and aged samples. The majority of fruit extracts yielded a distinctly different colour change compared to the purple that was produced from semen except for mushroom which also turned purple. Absorbance spectroscopy was used to determine the rate of the reaction at 525 nm. There were no significant differences between the rate of reaction for fresh and aged samples using both direct and indirect testing.