122 Forensic dentistryDNA sample is physically untouched and that detection can be made even in
the absence of a patterned injury.^46
It is the cellular component of saliva that makes it an attractive target for
the DNA analyst. These cells are not secreted by the salivary glands but are
incorporated into saliva as part of the shared oral environment. Specifically,
oral mucosal cells are sloughed into the salivary mix through normal epithelial
turnover and the activity of mastication. Additionally, white blood cells, most
commonly the acute inflammatory polymorphonuclear leukocytes, arise from
the crevicular fluid secondary to gingivitis. The microbial fauna that perpetu-
ally occupy the oropharyngeal spaces carry their own genomes and add to the
total sum of DNA, but in the context of forensic analysis, the human-specific
DNA primers do not recognize the bacterial DNA sequences.^46
The ability to obtain forensic DNA data from salivary evidence was well
described by 1992, but it was not until Sweet et al. published the double swab-
bing technique in 1997 that the standard of practice for salivary DNA col-
lection from skin was truly established.52,53 The double swab procedure takes
into consideration the need to moisten and lift the cellular component of an
adherent saliva stain as well as addresses the risk that the bitemark victim’s
skin will be a likely contributor to the DNA in the sample. The technique
requires two sterile cotton swabs and 3 ml of sterile, distilled water.^53 A brief
description of the procedure follows:
- Thoroughly moisten the head of a cotton swab in sterile, distilled water.
- Roll the head of this swab over the area of the saliva stain while using
moderate pressure and a continuous circular motion. - Allow this first swab to air dry in a contamination-free environment
for at least thirty minutes. - Within ten seconds of completing the first swab, roll the tip of the
second, dry swab across the now moist area of the stain. - Use a circular motion and light pressure to absorb the moisture from
the skin into the second swab. - Allow the second swab to air dry in a contamination-free environ-
ment for at least thirty minutes. - After drying, both swabs are packaged together, sealed, and marked
with unique sample and case numbers. - The chain-of-custody document is completed and samples are sub-
mitted to the laboratory.
As an addendum to the sample collection from the victim’s skin, a DNA
elimination sample should also be collected from the victim to allow the lab-
oratory to subtract that profile from any data generated from the skin swabs.
This reference sample can be whole blood, a buccal swab (see below), or if
the victim is deceased, a tissue sample taken at autopsy.^53 The double swab