Question: what factors affect your results or affect or restrain you from gainig your answers

Sue Carney answered on 22 Jun 2011:

Great amaan7869aser! Glad you asked this. I saw your question just as chat was closing and it’s a really good one, so I didn’t want to miss it.

The first important thing to point out is that we don’t always find the evidence we’re looking for. That’s not because we haven’t done a good job, but might be because either the alleged crime didn’t happen or because the evidence has been lost before the exhibit was recovered.

Let’s consider an example: Imagine someone has been bitten during a crime. We don’t know who did it but we want to look for their saliva on the victim’s skin, where they were bitten, to see if we can get a DNA profile to try to identify the offender. The problem here is that body fluids on skin are gradually rubbed off — either onto clothing or onto other surfaces that the skin comes into contact with. I’d usually expect that to happen in a few hours, under normal circumstances. If we can recover the clothing worn over the skin, we can test that for saliva instead, but if not, we have to swab the victim’s skin very quickly after the crime to make sure we recover the evidence. Of course, that might not happen if the victim doesn’t report the crime straight away. The saliva will also likely be lost if the victim has a bath or shower. It’s the same for lots of other types of forensic evidence. They can be lost over time and how quickly they are lost depends on all sorts of other factors such as activities like washing, how much saliva (or other evidence) was deposited in the first place and where exactly the evidence was deposited. We call this length of time that evidence remains its persistence time. Persistence is very important to consider when deciding which tests we’re going to do, and persistence applies to other evidence types too, not just body fluids and DNA.

Other things that might affect our results are the limitations of the tests we use. Some of the tests are presumptive. That means that they give an indication that a substance is present but do not specifically identify that substance. The most common chemical test for blood is presumptive. That means that things other than blood can also give a reaction in the test. We need to be aware of what they are, so that we can rule them out before deciding that what we’ve found is blood. Luckily, blood looks pretty distinctive, so that’s not usually an issue. Interestingly (well I think so anyway!), one of the other things that gives a reaction in the presumptive test for semen, is cauliflower! It’s pretty obvious though, if we’re testing for semen and we find sperm, that we’re not dealing with cauliflower!

Other tests are limited by how sensitive they are. In other words, how much of a substance do you need to get the test to work. There’s a big variation in how sensitive different tests are. The presumptive test for blood is so sensitive that it can detect a single drop of blood in a bucket of water, or so we are told — I’ve never tested that claim. The DNA profiling tests we use are also pretty sensitive these days. Earlier versions of DNA profiling from the early 1990s needed lots of body fluid to make them work — such as a blood stain the size of a 10p piece. More current developments mean that we need much less material. Now we can get a DNA profile from a blood stain approximately 1mm in diameter. Thats about the size of a pin head, or maybe smaller.

The fact that DNA profiling is now so sensitive leads to other limitations. With our most sensitive test, we can try to get DNA profiles from things that have been touched, without there being any visible biological material there at all. If we do get a DNA profile from one of those items, it’s very difficult to then go on and answer more questions about how that DNA might have got there. It also means that we’re less sure that the DNA we’re detecting actually relates to the crime. It could be a remnant of DNA deposited on the exhibit purely innocently at some other time, or it could be contamination, so we have to be very careful in our interpretations.

So to summarize: Time (persistence), other activities (like washing), limitations of tests such as specificity and sensitivity, and interpretational issues relating to very sensitive tests, are all factors that might prevent the forensic science from giving a clear answer.

I hope that helps. If not, please do ask another question! 🙂