How to Clean DNA Off a Knife

How to Clean DNA Off a Knife

by Dr. Hsien-Hsien Lei
Posted November 21, 2007 in DNA and the Law

knivesAs most of us are now aware from many episodes of CSI, you can get DNA off of most anything. Because this is the case, if you’re contemplating a crime, you should think twice. Chances are, DNA can catch a thief. And a murderer.

Three suspects accused of murdering British exchange student Meredith Kercher in Italy thought they’d gotten rid of the evidence by cleaning the alleged murder weapon, a kitchen knife, with bleach. But it wasn’t good enough. DNA could still be recovered from the tip and handle of the knife.

Juliet Lapidos, Slate’s Explainer, says bleach is very effective at destroying DNA but you have to be very thorough.

So, why did Knox and Sollecito’s bleaching gambit fail? It’s difficult to swab a knife thoroughly. Dried blood can stick to the nooks and crannies in a wood handle, to the serrated edge of a blade, or become lodged in the slit between the blade and the hilt. With help from a Q-tip, it’s possible to eliminate most stains, but what’s not visible to the naked eye might still be visible to a microscope, and sophisticated crime labs need only about 10 cells to build a DNA profile.

DNA specialist Dr. Karen Rudolph has been able to extract one speck of dried blood from the hinge of a folding knife that had been boiled. The suspect was later convicted.

This all begs the question: Why don’t the suspects destroy the murder weapons rather than cleaning them? If there’s enough time to bleach and boil, surely there’s enough time to find a way to dispose of the knife. Not that I would know, of course!

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Comment by Matt Mealiffe Subscribed to comments via email

I’m a big fan of presuming innocence until an individual is proven guilty in a court of law.
Hmmm…maybe I’m missing something, but is it really so surprising that the DNA of two people that live together is found on a kitchen knife in their apartment? Just a guess, but I bet my DNA is on a lot of kitchen knives in my home.

Matt, I have no idea if those three people involved in the Italian murder case are truly guilty. The media, of course, is painting them as such….

Comment by FactsOnly Subscribed to comments via email

Hey Matt,
The knife was NOT found in the home the victim and accused both live in. It was found in Knox’s boyfriends house. Knox’s other roommates confirmed it was not a knife thet ever saw in their home. When asked about the knife and the victims DNA, Knox’s boyfriend replied that he accidently pricked her while cooking together at his house. AK and others confirmed the victim was NEVER at the boyfriends house. Things are a bit different now, huh?

Comment by Anastasia

But the knife came from Rafaelle Sollecito’s apartment, Amanda’s boyfriend, so why would her boyfriend have a knife with Amanda AND Meredith’s DNA? Especially troubling is the fact that Meredith had never been to Rafaelle’s place.
If anyone wants to find out more about this case check out the Url to ‘Perugia murder file’

Comment by David Bradley

Maybe they should’ve used boiling bleach and then some scalding hot aqua regia, that would not only have done for the DNA on the knife but the fumes would’ve done for them too.


David, I was thinking the same!

Comment by L.A. Jones Subscribed to comments via email

Bleach ? What about Hydrogen peroxide? I believe H3O is the professional way to remove blood from just about everything.

Comment by Bob Subscribed to comments via email

Matt, unlike CSI, offenders do not break down and admit everything as soon as the most tenuous of evidence is presented in front of them. It’s fair to say that DNA left legitimately is going to be less valuable evidentially (but enough for the media to sensationalise) in court and isn’t going to break the case.

But if DNA from both suspect and victim are on the knife, one is Celluar DNA (from touching, handling) and the other DNA from a blood stain, then it puts a different slant on things.( I don’t actually know if this the case with regards this incident but you get the idea).


Comment by Subscribed to comments via email

I do not think that the case against Amanda Knox and her boyfriend is strong enough yet. The knife could have been brought from her boyfriend’s home to her home by her, and then was used in the murder and washed by someone else, and then was brought back to her boyfriend’s home by her again. As to the surveillance video showing her coming back to her home in light clothing, it can explained away by pointing out that the video might have failed to record her leaving the house in dark clothing soon afterward, before the murder occurred.

The only thing that is difficult to explain is the fact that she changed her stories and they bought bleach to clean the house the day after the murder occurred.

Comment by jhay

That’s why those criminals get caught. They don’t think things through. If they really gave it a thorough thought, they could’ve chosen not to do the crime from the beginning because they are well aware of how sophisticated the real CSI could be.

Comment by Gorilla

if you were really smart you would have the bleach bought way ahead of time so that they cannot trace the “pure coincidence” of it being bought very close to the murder. When cleaning the knife wear THICK THICK rubber gloves so as to not get your finger prints on again. Dont wash just once. Wash once as through as you can and let it soak in the bleach for a few hours and then wash again. Sure it might rust but its better then being in jail right? There are other more effective chemicals than bleach tho but you would need some real connections to get those and they wouldnt damage the knife…just the dna

Comment by L.A. Jones Subscribed to comments via email

use H3O hydrogen peroxide.

Comment by Anastasia

The thing about the Kercher murder case is the fact that the double DNA knife was found at Amanda’s boyfriends place, it was determined to be his knife, because Amanda’s two other flatmates, Laura and Filomena, testified in court, that the knife found at Raffaelle’s was not one of their households knives. Plus Meredith (the victim) had never been to Raffaelle’s place, so why would Meredith’s DNA show up on the tip of the knife? Its obvious why Amanda’s DNA would be on the knife, she his girlfriend.
I tried to post a comment earlier but it hasnt showed up yet, so I am posting this again.
Also if anyones wants the facts on this case and a good place to discuss them openly, check out this link.


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