DNA on the knife

Regardless of the DNA questions below, a trial and a jury is a human process. Apparently the jury initially only heard the defense claim of contamination when the DNA was presented: from quotes in the press I’ve seen, when the prosecution presented its case, “The defence teams’ forensic experts are not disputing that Meredith’s DNA was on the blade of the knife. Instead they are arguing that the knife was somehow contaminated for the DNA to actually be there.”

Thus the first question put in the jury’s mind may have been a question of the professionalism of the scientists and the police, NOT that the results of the testing were questionable.

At some point, Sollecito wrote in his prison diary an explanation that the victim was over his house for dinner one night and he accidentally cut her cooking. Thus, the next thing the jury may have heard was in fact a confirmation (even if it provided an alabi) that the DNA was the victim’s.

If the defense then turned around and tried to claim it wasn’t the victim’s, based on the science.. it presents a conundrum.

The DNA was the victim’s, and according to Sollecito it was for an innocent reason. But then the defense argues that it may not have been the victim’s, which then turns inpugns Sollecito, who then have to question his motives in coming up with the story in the first place.

The defense’s strategy and Sollecito’s explanation may have been more damning to the defense’s case then the DNA on the knife.

Add that to the conflicting stories between Sollecito and Knox as to whether or not she was at his house all evening…

Below are some references to the knife and the Low Copy Number (LCN) testing of DNA. (The LCN test was necessary due to the small amount of DNA pulled from the knife. ) Without direct access to the prosecution’s testimony (or a suitable translation) its hard to discern whether bloggers & news site statements on the testing procedures are based on primary or secondary evidence- i.e. what exactly was the procedure that was done in the testing.

The two biggest questions on the procedure seem to be that of doing a ‘test case’ to ensure the machine was clear before doing the LCN on the knife dna, and the method of ‘amplification’ the scientist used. One  theory was that known DNA may have been tested prior to the LCN test on the knife, thus producing the false results. There is also a question on the ‘amplification’ used; one of the sites below claims the ‘amplification’ was akin to an adjustment to the magnification on the machine used.

These could easily be resolved by the answers to a few questions-

1-was the machine used calibrated to only go down to, say 100 RFU?

2-was the ‘amplification’ a mechinical procedure or a scientific testing procedure?

3- What is the log of DNA tests that day?

4- a semi-related question I’ve seen; what other parts on the knife blade were tested to see if they produced results?

Apparently a few american scientists did a review of the victim’s DNA found on the tip of the knife entered into evidence; written up in this article:


The article quotes the letter as stating:
“To minimise the risk that some peaks arise from contamination, most US labs only count peaks above a height threshold of 150 relative fluorescence units (RFUs) and all dismiss those below 50. The trouble with the DNA found on the knife is that “most of the peaks are below 50″, says Greg Hampikian of Boise State University in Idaho, who signed the letter and reviewed the DNA evidence”

“When this happens, samples can be rerun, but this doesn’t appear to have been done in the Knox and Sollecito case. This means contamination cannot be ruled out, the open letter claims. The same lab may also have been running DNA profiles from other evidence in the case at the same time, it says, and tiny amounts of this could have contaminated the knife samples.”

(this letter was previously on scribd, but has been removed:


However, I ran across this statement from a case in 2006 in Dever, CO (USA)

“Lisa Calandro was the DNA laboratory supervisor in the Forensic Science
Division at Forensic Analytical, a private forensic and environmental testing and consulting firm in Hayward. She testified as an expert for the prosecution in forensic DNA analysis.

Calandro’s laboratory uses 100 RFU as a “normal detection limit,” although
analysts will go as low as 50 RFU if it is fairly certain there is a peak. It would be necessary to be somewhat cautious about any interpretations below 100 RFU since, the lower the RFU, the greater the possibility of detecting electronic “noise,” rather than a true peak. The lower the RFU number, the higher the amount of data that can be interpreted. But there is also an increased probability of detecting “artifacts,” i.e., not real DNA.”

Greg Hampikian in signing the letter from the American scientists reviewing some of the DNA evidence states:

“Almost all peaks are below 75 RFUs, and most are below 50 RFUs.”



However, in December, 2007, a conviction on a case in Britain was overturned, in part on the argument that LCN (Low Copy number) was unreliable


Two articles at another site talk about LCN and its problems:


This article at the same site by Mark Waterby specifically looks at the DNA in the knife and LCN,


It references a description of the ‘testing’ method on this site:


“It didn’t sort anything but Mrs Stefanoni didn’t give up and started to amplify and amplify until the first peaks appeared. The machine was not allowed to go beyond, but something there was and had to be taken out. The goodwill scientist broke the seals and kept amplifying and amplifying and amplifying until, in a forest of background noise peaks, some alleles emerged. She decided which were the stutters, the false ones, and which the real alleles, et voila!”

The Mark Waterby article concludes

“The profiling that Stefanoni performed on the DNA from the blade of Raffaele’s kitchen knife meets either of these definitions. There was so little DNA present that the instrument indicated no DNA until Stefanoni overrode the machine limits.”

One commenter claims

The prosecution’s forensic scientist knew what suspect patterns she was trying to match, and discarded high peaks from the crime scene samples that didn’t fit the suspects’ DNA. Ms Stefanoni accepted low peaks (test system noise) that should have been discarded, but which happened to match part of the suspects’ DNA patterns. So she matched the answers – to the test, and selected results that benefited the prosecutor’s case.

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