Regarding the question that came up previously about whether Stefanoni treated the knife differently than some of the other evidence. The answer to that question would be a youbetcha. From Amanda's appeal pages 61/62:
It is very clear that Stefanoni regarded this piece of evidence as very important in terms of the prosecution's case and went the extra 99 miles to find something, even if it was done without following normal standards, protocols, and guidelines. All this other evidence was not "useful" or "suitable" for further testing, yet the knife was.
One of the arguments of those who believe unconditionally in Knox and Sollecito's guilt is this: how can anyone simultaneously accept - uncritically - the DNA evidence against Guede, yet question the validity of the DNA evidence against Knox and/or Sollecito and the kitchen knife? After all, the DNA testing was all conducted at the same laboratory by the same person etc etc.
The answer to this alleged conundrum is twofold:
context, and
volume.
Context:
Firstly, Guede's DNA should not have been in the girls' house at all, since he had no innocent reason for it being there. However, if his DNA had been only found in the bathroom or the lounge, he might have been able to explain it away innocently. Unfortunately for Guede, though, his DNA was also found inside the body of the murder victim, upon her handbag (purse), and on her sweatshirt. This multiple presence of his DNA in very incriminating places points heavily towards culpability.
Conversely, the presence of Knox's DNA inside the house where she lived is hardly incriminating. The so called "mixed DNA" samples recovered from the small bathroom are questionable, since they were not compared with reference samples (e.g. from elsewhere on the sink/bidet tap or plughole), to see whether Knox's background DNA was also present there. There was no Knox DNA found inside the murder room or upon the victim's body or clothing.
Sollecito had only been to the girls' house a small number of times, so clearly there was less chance of his DNA being there via an innocent route than with Knox, but still far more chance than with Guede. In fact, his DNA was only located in two places in the girls' house: on a cigarette butt in the lounge, and on the clasp of the victim's bra. But this leads on the the second factor.....
Volume:
Guede's multiple incriminating deposits of DNA were found in relatively large volumes, and could all be detected by standard PCR testing procedures (with which the DNA lab and the tester were both familiar and competent).
Knox's DNA was found in low levels (but still within standard PCR ranges) in the bathroom swabs - but, as discussed above, the police failed to check whether Knox's DNA was present in these low levels elsewhere on likely surfaces in the bathroom which she used daily.
Sollecito's single incriminating deposit of DNA was only found in very small levels on the bra clasp (levels which fall below the threshold for LCN classification). So, arguably, the laboratory was not properly qualified to test at these low levels (which require special air-handling facilities and extremely high levels of sterility).
And, of course, the incriminating DNA of the murder victim identified on the blade of the kitchen knife was deeply within LCN territory, so the lab was certainly unqualified to conduct testing and verification of such minute levels of DNA matter.
So this is why the DNA evidence against Guede has validity: his DNA shouldn't have been anywhere in the girls' house, let alone in several places in the murder room, upon the murder victim's clothing and belongings, and inside the murder victim herself. And his DNA was apparently present in relatively significant quantities, meaning that the testing procedures can be assumed to be relatively accurate and reliable.
