When there is a an injection site found this what happens...
The cell area around the injection site would be more sterile that other areas on the body.
They take skin cell scraping from around the immediate injection site, then they take skin cell scrapings from area near the injection site (from a outside an area that is at least 2" in diameter away from the injection site)
These skin cells are then examined against each other. The skin cells coming from the area right at the injection site should be more sterile if a alcohol swab was used.
Then they also take tissue and muscle samples from the area under the injection site to determine what drug was injected in to that area.
I figured their had to be a reson for him asking for this. I just assumed that they could pick up alcohol residue or somthing from the skin.