Toluidine blue is a dye. It is used to stain specimens for microbiologists and other scientists. It is also used to stain living tissue for several reasons. One reason is to screen for cancer, since precancerous cells and cancerous cells take up more dye that normal cells. In this application I have seen references to oral cancer screening and vulvar cancer screening. These are uses I am only familiar with from reading on the web and are not really relevant to my studies.
In the context of a sexual assault forensic examination Toluidine blue is used to locate and document injuries. Because the dye is selectively taken up by injured tissue (stains “identify occult abrasions by binding with nuclei in intraepithelial skin, vulvar skin contains no nuclei”). Micro abrasions and lacerations can be visualized after the genital and perianal area are stained with Toluidine blue and then destained. Any remaining blue after destaining is indicative of cellular damage and may be due to injury from a sexual assault.
Tips about using Toluidine blue as part of the rape kit:
Only stain the external vulva, most especially the posterior fourchette, do not stain mucous membranes.
Although dilute acetic acid (vinegar) is the frequently recommended destaining agent it hurts when sprayed on injured tissue. Surgilube works just as well and doesn’t hurt.
Always perform the Toluidine blue exam prior to any other touching of tissue digitally or with a speculum as these touches may cause distracting micro-injuries.
Assure the patient the slight bits of remaining blue left after destaining will fade within a few days (if they are concerned).
Avoid spills, put the bottle down after use in a waterproof container.
Have the pharmacy mix the powder to a 1% solution with sterile water. They need to do this under a laminar flow screen, as inhalation of the powder is dangerous.
Less is more.
Toluidine blue and the destaining agents do not interfere with DNA identification of recovered semen.
The use of Toluidine blue has been found to “increase the detection rate of posterior fourchette lacerations from 16% to 40% in adult rape victims”. Yes, it is messy and the patient and examiner both usually hate it but it is worth doing.
Despite the value stated above it is good to remember that we do not yet have research about the percent of micro injuries that may or may not occur during consensual sex. This is currently being researched and will be valuable to know when testifying in court when/if results are published in a peer reviewed journal.
class lecture notes
Forensic Emergency Medicine – textbook by Olsahker, Jackson, Smock