If there is one or more unidentified persons who contributed their DNA to a forensic sample, one cannot be at all confident, without strong corroborating evidence, that any other "suspect" DNA on that sample was
not the result of contamination.
There was absolutely no corroborating evidence to indicate that Raffaele Sollecito's DNA, nor that the DNA of any of the other unidentified males, allegedly found on the bra clasp, had any connection or relevance to the crimes against Meredith Kercher. There was evidence to the contrary: 1) the bra had been forcefully torn off Kercher's body; 2) the DNA of Rudy Guede was found on the bra fabric, Kercher's clothing, her purse, and in the rape kit; 3) one does not (typically) touch the clasp to fasten or unfasten a bra; 4) the clasp had been collected 46 days after the other evidence items, from the floor after having been moved, apparently during non-recorded police activity, and was improperly collected by investigators with dirty gloves, as recorded by the police; and 5) the clasp DNA testing did not meet international standards - there was no evidence of the proper DNA-negative controls and there was an irregularity in DNA sample numbering suggesting tampering by the scientific police.
With the exception of the obviously contaminated bra clasp, the DNA of no other male except Guede was reported by the police to be in the murder room or in or on Kercher or her clothing or purse.
DNA is of course present in epithelial cells. There are many references providing information on the topic of the forensic testing of touch DNA from epithelial cells, for example:
https://www.forensicmag.com/article/2013/04/touch-dna-crime-scene-crime-laboratory
Some relevant excerpts from the above source:
"Touch DNA
Epithelial or touch DNA evidence can be defined as evidence with no visible staining that would likely contain DNA resulting from the transfer of epithelial cells from the skin to an object. Can simply touching an object leave skin cells? It has been stated in publications that forensic scientists can obtain a DNA profile from as few as five to six cells. However, just because a surface is touched and a few skin cells are left behind does not guarantee a meaningful DNA profile can be obtained. Detecting and obtaining an interpretable DNA profile are two different concepts.
The touch DNA testing performed by most public crime laboratories utilizes the same STR typing method performed by DNA analysts for the last 10–15 years. It is not a new technology. There are alternate methods of detecting DNA from a smaller number of cells ranging from 15–30 diploid cells. This is known as Low Copy Number (LCN) testing and utilizes “enhanced” techniques for obtaining DNA profiles. The experts and courts have had disagreements on its validation, interpretation, use, and acceptance. Typically, STR typing is known to require approximately 70–150 cells to produce a DNA profile."
"Contamination is the unintentional introduction of outside DNA into a crime scene or laboratory sample. Contaminant DNA may appear as background DNA, the major or minor profile within a mixture, a single source DNA profile, or all of the above. When can this occur? Before the commission of the crime, after the crime and before the crime scene is discovered/secured, during the crime scene investigation, and within the crime laboratory or DNA laboratory. (Figure 1) In today’s world of touch DNA, a crime scene has to be approached in a way to minimize contamination since one cannot see or test for touch DNA."
"The DNA Lab and Contamination
Once evidence is received into the DNA laboratory, how is contamination minimized and what are the potential sources of epithelial cells? The Quality Assurance Standards for Forensic DNA Testing Laboratories addresses DNA contamination in Standard 9.7. In the discussion section it states, “A laboratory shall have and follow a documented policy for detecting and controlling contamination. This policy should include the procedures used by a laboratory for monitoring, decontaminating, and detecting contamination. In addition, a laboratory shall have and follow policies and/or procedures for interpreting data potentially affected by contamination.”2
In addition to lab coats and masks, how is contamination detected, controlled, and monitored? Some examples which may be employed are:
Access to the laboratory shall be controlled and limited to prevent access by unauthorized personnel.
Separation of the work areas for evidence examination, DNA extraction, and DNA amplification and typing.
Exhibits are examined at different locations and/or times.
Negative controls are being used from evidence examination through extraction and amplification.
Contamination logs monitor introduction of exogenous DNA and identify sources.
Elimination profiles of past and present DNA analysts or support staff are maintained.
The European Network of Forensic Science Institutes (ENFSI), the FBI Laboratory’s Scientific Working Group on DNA Analysis Methods (SWGDAM) composed of U.S. and Canadian members, and the Australian Biology Specialist Advisory Group (BSAG) published a joint position statement advocating that forensic DNA laboratories maintain an elimination database for screening DNA results as appropriate."
Stefanoni did not follow, or refused to provide records and data substantiating, the laboratory procedures used to detect, control, or monitor contamination. It was thus certain that she did not follow the
international standards for contamination detection, control, and monitoring.
She also did not provide the replicate (duplicate) DNA profiling required for the bra clasp sample and the knife blade sample claimed to show Sollecito's DNA and Kercher's DNA, respectively.
