Blood, as most of you know, is a great source of DNA. Whenever there’s blood at a scene there’s a good chance of catching the offender, or in some cases identifying a victim. Blood has been glamourized by television, and is the centre of shows like Dexter. I’ve seen how characters in these programmes take swabs of blood and parts of it are accurate, but some aren’t. I will go over the process of what I do when I swab blood, and explain my actions.
Here we have a red stain on a window. There’s been vandals in the area and, if this stain is blood, it will tell us whose been hanging around here. Our Detectives can then interview the person and ask for reason’s why their blood was found. I’ll do my photography first - taking long, mid-range and close up shots of the blood.
Then we’ll want to see if it is blood. I’ll start by taking a very small sample of the blood using a piece of filter paper. I’ll fold the paper twice to create a point, and rub it against the edge of the blood stain.
I have two chemicals I use as a presumptive test. On the television you normally see them use just one chemical, and the blood turns a pink colour. For me the test should turn the same pink colour, but only after I add the second chemical. I use Kastle Mayer and Hydrogen Peroxide, which reacts at the presence of hemoglobin. Kastle Mayer is dripped onto the filter paper first, followed by Hydrogen Peroxide. I always remembered which one was first by linking it with a brand of peanuts: K comes before P, Kastle before Peroxide, KP nuts!
This is only a presumptive test and not a definite indicator, so you need to allow for some discretion. I’ll then take a swab of the blood stain using a wet swab. I use a small capsule of sterile water and drip it onto the first ‘wet’ swab. I’ll load up the swab with a decent amount of blood.
I normally take a dry swab too, going over where the blood was to mop up any of the stain left behind. Some SOCO’s choose not to do this and rely on their one wet swab, but it’s just my preference. There’s no right or wrong.
‘Control swabs’ are swabs without any blood/crime-stain on, but are taken so the Scientist knows what background contaminants are on the main swab. For example, you may take a swab of blood from somewhere heavily contaminated with skin cells. I’ll take a background control swab of the area away from the blood stain, so the Scientist can identify what should and shouldn’t be on the main blood swab. It’s a bit like spot the difference - the difference between the background control and the main wet swab should only be the crime stain (in this case blood). This will always be a wet background swab, so there’s a control sample of the water too.
A ‘batch control swab’ is always retained at our main Forensic base. This is a plain, unopened swab from each batch. If a batch of swabs has been contaminated when being manufactured, the batch control swab should be able to tell us.
So if you ever see me swabbing a crime stain and it looks like I’m missing it don’t worry, I’m probably taking a control swab.
The swabs will then get sealed into an evidence bag, ready for me to submit to an Independent Forensic Provider, and hopefully help close the investigation with a positive result.