Method of Sampling

Two samples must first be obtained from the perianal area. Just as when sampling the skin elsewhere, if the perianal skin is moist, the stain should be retrieved on dry swabs. If there is no visible staining or the stain is dry, the double-swab technique should be used (28). The forensic practitioner should use as many swabs as are necessary to remove any visible stain (repeating moistened swab followed by dry swab). If no stain is visible, two swabs will suffice (the first wet, the second dry). Although not specifically defined for forensic pur poses, the perianal area should be considered as an area with a radius of 3 cm from the anus. The swabs are then placed in sheaths without transport medium. Even though traditionally these swabs have been labeled "external anal swab," they should be labeled as "perianal swab" to clearly indicate the site of sampling. The anal canal is then sampled by passing a wet swab and then a dry swab, sequentially, up to 3 cm through the anus. The proctoscope (anoscope) is then passed 2-3 cm into the anal canal, and the lower rectum is sampled using a dry swab. As the proctoscope is withdrawn, the anal canal can be sampled, again with a dry swab. As discussed previously, when examining female complainants of anal intercourse alone, swabs should also be obtained from the vagina.

The best practice is to use only sterile water to lubricate the proctoscope, because research has shown that swabs that are contaminated by some lubricants yield significantly less DNA, and lubricants may have been used in the incident (Newton, M., personal communication, 2003). In practice, this presents major difficulties. If doctors decide for clinical reasons to use a lubricant, they should apply the lubricant (from a single-use sachet or tube) sparingly, taking care not to contaminate the swabs, and must note its use on the forms returned to the forensic scientist.

In the process of sampling the rectum/anal canal, the proctoscope may accumulate body fluids and trace evidence. Therefore, the used proctoscope should be retained, packaged separately, and stored in accordance with local policy. If the proctoscope is visibly wet on removal, swabbing may be conducted to retrieve visible material. If storage space is restricted, then the instrument should be swabbed and the swabs retained instead.

Stool samples and toilet paper need not be collected routinely because the other samples described should be adequate for laboratory requirements.

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