The comments in this section refer to nongenital skin. For genital and perianal skin, see Headings 8-10.

4.1. Forensic Evidence 4.1.1. Method of Sampling

All areas of unwashed skin that have been licked, kissed, sucked, bitten, or ejaculated on by either the assailant or the complainant must be sampled. Cellular material, amenable to DNA profiling techniques, has also been identified where there has been skin-to-skin contact (e.g., manual strangulation or gripping the arm) ([24] and Burgess, Z., personal communication, 2001). Therefore, when dealing with an assault conducted by an unknown assailant, consideration should be given to sampling marks or injuries on the skin that the complainant attributes to direct contact by the offender. However, the problem with this type of sampling is there is considerable lack of understanding about issues of transfer and persistence (24). Consequently, speculative skin swabbing in the absence of visible marks or injuries is not recommended.

Although several techniques, including the use of surgical gauze pads (25) and cigarette papers (26), have been employed to recover saliva and other trace evidence from the skin with variable success, the use of sterile swabs is the most widely used technique that has received international endorsement (27). If the skin appears moist, the stain should be retrieved on dry swabs, which are then placed in sheaths without transport medium. The double-swab technique, described by Sweet et al., is the recommended method to recover dried stains or possible cellular material from skin (28). When using this technique, sterile water is used to wet completely the cotton tip of the first swab. The tip of the swab is then rolled over the area of skin using circular motions while rotating the swab on its long axis to ensure maximum contact between the skin and the swab. Then, a second dry swab is rolled over the same area to absorb the water left on the skin by the initial swab and collect any remaining cells. Minimal pressure should be applied to prevent exfoliation of the patient's own epithelial cells. The forensic practitioner should use as many swabs as necessary to remove any visible stain (repeating wet swab followed by dry swab). If no stain is visible, two swabs will suffice (the first wet; the second dry). The swabs are then placed in sheaths without transport medium.

Some authors comment that ultraviolet (UV) light causes fluorescence of semen and saliva and advocate its use in determining the areas of skin to be swabbed (29,30). This advice must be interpreted cautiously, because a study by Santucci and colleagues found that although many creams and ointments fluoresced when exposed to a Wood's lamp (wavelength 360 nm), none of the 28 semen samples examined did (31). In addition, other authors have commented that detergents, lubricants (particularly those that contain petroleum jelly), and milk also fluoresce (32).

However, when semen stains are exposed to a high-intensity light source of variable wavelengths (e.g., the Polilight®) and viewed using goggles to block the strong excitation light, then semen may be detectable, even when the background surface is fluorescent (33). Furthermore, the location of the stain may be recorded using photography.

A recent experiment by Marshall and colleagues found that semen from a single donor could be detected on skin using several excitation wavelengths (emitted by a Poliray®) and emission filter combinations (34). Optimal results were obtained using 415 nm ± 40 nm band-pass filter and a 475 high-pass and 505 band-pass ± 40 nm interference filter. More research must be conducted using semen from multiple donors and isolating semen from other fluorescing contaminants, such as oils.

Furthermore, Nelson and Santucci have recently described training forensic physicians to use an alternative light source (the Bluemaxx BM500) to identify semen (100% sensitivity) and to differentiate it from other products (35).

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