Within spitting distance – is saliva a significant forensic evidence type?

Saliva bubbles coming from a baby's mouthWhilst blood and semen are the two most commonly encountered body fluids, saliva can often arise as a significant forensic evidence type, for example:

  • on a mask worn during a robbery can help to identify the offender
  • drinking vessels or cigarette ends at scenes of crime
  • genital swabs in a sexual offence
  • upper areas of clothing where it can help identify the wearer of the garment

However difficulties occur because saliva cannot be easily identified. The test to identify whether it is present  relies on the detection of a substance called amylase which is usually present in large amounts in saliva.  Complications arise because amylase also exists in other body fluids although with the exception of faeces and vaginal secretions it is usually present at much lower levels than in saliva.

Consider a face mask where a strong amylase result has been obtained. This would be a good indication that saliva is present since other body fluids are unlikely to be present. In this situation you may see the forensic scientist report ”in my opinion these results are due to the presence of saliva”.

It is more problematic when a weak amylase result is obtained or there is the possibility of other body fluids being present. For example, a weak amylase result on a penile swab where vaginal secretions might be present. Here the scientist may use phrases such as:

  • “tests for the presence of saliva were inconclusive”
  • “reactions indicative of the possible presence of saliva were obtained”
  • “it was not possible to determine whether saliva was present”

Another issue is that some people have low levels of amylase in their saliva so a negative result to the test does not necessarily mean that no saliva is present.

 

Can a DNA profile be obtained from a saliva stain?

Saliva usually contains cells sloughed from the lining of the mouth; these cells contain DNA and so DNA profiling can be used to associate the stain with an individual.  However it is not such a rich source of DNA as blood or semen and the chance of successfully obtaining a DNA profile from saliva is variable.

Although a DNA match is powerful evidence, in many cases the significant issue will be how was the DNA transferred, and whether it came from saliva or another biological substance such as cells transferred by touch. There may a legitimate reason why the individual’s DNA or saliva is present.

 

If there is a DNA match, does it matter whether the stain is saliva?

Sometimes.

In many cases, the DNA match and how the DNA was transferred will be the significant factors, however in others whilst there may be a legitimate reason why a person’s DNA is present, the presence of their saliva would be significant. In this situation it is important to consider firstly, has the forensic scientist given an opinion about whether the stain is saliva,  and secondly have they explicitly attributed the DNA profile to saliva?

 

Some case examples 

Significance of DNA matching the complainant on the defendant’s penile swab

The complainant made an allegation of fellatio which the defendant denied, however he said that he did insert his fingers into her vagina.  DNA matching the complainant was detected on the defendant’s penile swab.  Initially this may appear to support the Crown’s case. However if the DNA has not been specifically attributed to saliva, its possible that the DNA has come from vaginal secretions or cells deposited by touch. If the defendant touched the complainant’s vagina or skin, he may have transferred her DNA to his penis via his hands.

Saliva on clothing

It is not unusual for saliva to be present on a person’s clothing – whilst talking, shouting, sneezing etc. we can naturally spit out small droplets of saliva onto the front our own clothing and any nearby surfaces.  Such natural transfers of saliva can become evidentially significant.  Recent research around the effectiveness of wearing masks to reduce Covid19 transmission has highlighted how easily individuals disperse saliva when talking, sneezing and coughing.

Consider a sexual assault in which the assailant grabbed a woman from behind and then raped her. No semen or blood was found on the complainant’s clothing or intimate swabs.  As her assailant had held her from behind and had talked to her during the offence the back of her top was examined and showed an area of possible saliva staining.  A DNA profile obtained from this staining allowed the incident to be linked to a series of crimes and subsequently led to an arrest.

The arrested man said that his saliva must have been transferred onto her clothing whilst he was standing near her on a crowded train. The forensic scientist can assess the pattern and distribution of staining given the two accounts, however it is never possible to determine when any saliva staining was deposited, other than to say that it was since the item was last washed.

Saliva as evidence of suffocation?

A woman was found dead at her home, but the cause of death could not be ascertained.  It was suggested that she had been smothered using a cushion from the sofa.  Amylase testing indicated that saliva was most likely present, and a DNA profile matching the deceased was obtained.  However, the significance of the saliva was questioned – it could have been deposited by sleeping with her head on the cushion at some time and did not necessarily indicate that she had been smothered with the cushion.

 

If you encounter a case involving saliva evidence, please email or phone. We are happy to give you an initial view on whether it is worth instructing an expert to review the evidence for your client.