Effectiveness of STK Spray® for semen stain localization on solid surfaces: A specificity and sensitivity study

Semen identification is a crucial step in sexual assault cases. The aim of this study was to assess STK Spray®, a presumptive test for semen, under controlled conditions including, simulated crime scene stains detection. Easy to use, it can be sprayed directly onto different surfaces and visualized under UV light. Several tests were performed on five different substrates (ceramic tile, drywall, metal, wood, and faux leather). The spray was able to enhance semen fluorescence, especially in diluted samples, with characteristic "globular" spots. Although it showed good specificity, false positives could be obtained with 10% bleach. The fluorescence signals were quantified using ImageJ™ and showed a statistically significant substrate-dependent variability. Mixture analysis indicated that saliva did not interfere with detection of semen, while urine partially suppressed the signal and blood markedly affected its interpretation. Simulation tests with UV lamp comparisons confirmed the importance of choosing the right detection method and the utility of this presumptive test in combination with additional immunochromatographic tests. A preliminary signal retention test showed stable fluorescence for up to two years when stains were stored appropriately. Finally, complete DNA profiles (100% of alleles) were obtained from all samples (n = 24) after exposure to the reagent and UV light. Because of its ability to enhance semen signal, especially on specific surfaces, and its rapidity of use and detection, STK Spray® may represent a useful aid in the preliminary screening phase.