Update (11/7/2013)
Poy and Van Oorshot wrote, "To further help evaluate the above finding swabs were taken from gloves worn whilst examining a heavily soiled dress during routine casework examination. A significant amount of DNA was retrieved which exhibited a genetic profile that matched that of samples taken from the exhibit." This is direct evidence that gloves can transfer DNA.
Introduction
The issues of possible DNA
contamination and proper versus improper handing of evidence are at the center
of the forensic evidence in the trials of Amanda Knox and Raffaele
Sollecito. This entry will examine
two aspects of DNA forensics, dealing with airborne DNA and the need to change
gloves when handling a fresh item of evidence. Both kinds of potential problems need to be recognized and addressed in order to minimize the chances of contamination.
The problem of airborne DNA
Barbie Nadeau wrote, “…the
defense claims that the crime scene was badly compromised during the collection
of evidence. Alberto Intini, head
of Italy’s national forensic team, disagrees. On the stand, he defended the
forensics work and stressed that the crime scene had not been contaminated,
especially under cross examination when the defense lawyers tried and failed to
prove otherwise. ‘DNA does not fly around like pollen,’ he said…” Ms. Nadeau
left her readers with a misimpression.
Later in the trial the defense returned to this issue. “’DNA does not have wings, but it
flies,’ [defense expert witness Sarah] Gino cautioned. ‘In a laboratory where
hundreds of samples are examined, the risk of contamination exists and should
be taken into consideration.’”
Dr. Intini’s view is
seriously in error, as can be ascertained by a number of lines of
evidence. A paper on DNA in
fossils (BioTechniques 38:569-575, April 2005) notes, “These molecules are
easily spread via aerosol transport. One aerosol droplet can contain many more
DNA molecules than one gram of fossil material.” In a discussion of how to
collect DNA evidence Dick Warrington advised, “Next, you can prevent
contamination by wearing a mask, since you want to avoid coughing and sneezing
around the evidence you are processing.”
pipettors and pipet tips
Pipettes are devices used to
deliver small volumes of liquids.
They are used frequently when the polymerase chain reaction (PCR)
technique is used to amplify tiny amounts of DNA.
For that reason the use of aerosol barrier pipette tips is routine in labs
engaging in PCR. An application
note on pipetting explained, “Cross-contamination occurs if improper pipetting
causes splashes or drips. Even if pipettors are handled properly, aerosols can
be generated that contain DNA molecules and can contaminate the pipettor and
subsequent pipet products. This demands increasing efforts to reduce the penetration
of contaminants by means of filter tips.”
Gilson, a manufacturer of pipettes, supplies a technical bulletin that reads
in part: “For example, if a technician in a crime lab performs PCR on a blood
sample, cross-contamination between samples could result in an erroneous
incrimination, even if the technician changes pipette tips between samples. A
few blood cells could volatilize in the pipette shaft, stick to the plastic of
the pipette, and then get ejected into the next test sample. Modern
laboratories have taken account of this fact and are devoting tremendous
efforts to avoid this problem through the use of filter tips."
laboratory traffic
A number of forensic
guidelines are in place to protect against aerosol DNA from one source or
another. For example, “A ‘one-way traffic’ rule
is also observed in the laboratory, once the technician has entered the PCR or
the post-PCR rooms, they are not allowed to return to the extraction or pre-PCR
rooms until the next day or a complete cloth changing in order to prevent contamination
by aerosol particles.” The
problems only get more serious in the low template region of analysis. Keith Bedford said, “The way I am
speaking at the moment, we could probably detect DNA on this pad in front of
me.”
laboratory design
Laboratories are constructed
in a way to minimize the possibility of contamination due to air flow. In the article Setting Up a PCR
Laboratory.” Theodore E. Mifflin discussed how the design of the laboratory can minimize the chances of contamination: “Air handling. For extremely
high-performance PCR laboratories that will be involved with detecting
very-low-prevalence DNA or RNA molecules (e.g., infectious disease agents in
clinical samples), additional measures may be necessary to prevent
contamination from the air being recirculated between the pre- and post-PCR
laboratories. In this case, the air handlers need to be separate and the air
pressure individually adjusted in each laboratory. In the pre-PCR laboratory,
there should be a slight positive pressure compared to the air in the
connecting hallway. The post-PCR laboratory, in contrast, should be at slightly
reduced pressure to pull air in from the outside and thereby prevent escape of
amplicons from the completed PCR samples being analyzed inside the lab (Fig.
2). Finally, the air handlers for the pre- and post-PCR laboratories need to be
connected to separate air ducts, and each must lead to a separate location for
exhaust.” Dr. Mifflin’s main focus
is pathology, but his points about low levels of DNA are germane to low
template DNA forensics in that both situations use PCR to amplify very small
quantities of DNA.
What the police did and did
not do with respect to changing gloves
When the forensic police collected evidence at Ms. Kercher’s
flat, they did not change their gloves frequently. One can observe in a series of photographs a failure to change gloves over several minutes and
several evidence samples. Barbie
Nadeau reported, “Sollecito’s attorney, Giulia Bongiorno, stopped the crime scene
video several times to point out errors. For example, Stefanoni testified that
she had changed gloves according to official investigation procedures, but
Bongiorno stopped the crime scene video twice to show that Stefanoni’s bracelet
and the fold of her glove were exactly the same before and after the time she
claimed to have changed gloves.”
Ms. Nadeau recounts the same incident on p. 133 of Angel Face, her book
on the murder of Meredith Kercher.
Ms. Stefanoni’s views on
when gloves should be changed are found in the English translation of the
Massei report. On p. 203
Massei wrote that Stefanoni "specified" that gloves were changed
"every time an object was touched that was particularly soaked with blood,
and when it was obvious that the gloves would be soiled;" Based on pages 204-205, she appears to
believe that the presence of a
liquid is necessary to bring about contamination by touch.
What forensic experts say about changing gloves
On page 38 of John Butler's 2005 textbook Forensic DNA Typing, he wrote, "Use clean latex gloves for
collecting each item of evidence. Gloves should be changed between handling of
different items of evidence."
Dick Warrington is the author of some articles in Forensic Magazine and
is employed by a company which makes equipment for crime scene
investigations. He wrote, “If you
pick up one piece of evidence and then pick up another piece of evidence you
can transfer evidence from the first item to the second item. You can avoid
this kind of cross-contamination if you remember to change your gloves before handling each piece of evidence.” He also advised, “Put on gloves, use
gloves, change gloves. Do that every time you touch a piece of evidence.
Likewise, use disposable tweezers, scalpels, etc. Change these each time they
are used, as well.” Orchid
Cellmark’s guidelines for collecting DNA evidence read in part, “Use clean
latex gloves for collecting each item of evidence. It is recommended the gloves
be changed between the collection of each item of evidence.”
Conclusions
The need to use fresh gloves when handling a new piece of evidence is utterly noncontroversial. Yet it is obvious from video of the collection, that
the forensic police did not change gloves frequently. Their failure to follow the consensus
view of what is good practice might have contaminated the bra clasp and might
have led to mixed DNA samples elsewhere in the flat. The fact that airborne DNA can compromise an experiment is likewise the consensus view of practitioners of PCR. Dr. Intini’s ignorance of the dangers of aerosol DNA shake one’s
confidence in the ability of a forensic lab under his supervision to combat the
dangers of airborne DNA contamination even in routine DNA profiling, let alone
low template profiling.
