Module 11 Case Study Answers

Experimental Protocol

In this study, they wanted to mimic crime scene interactions, evidence handling, and evidence analyzing as closely to the real world as possible. They started by asking two different police districts their cleaning procedures and evidence handling.

Wipe test

Any look at contamination would be remiss without taking a first look into cleaning or, specifically, cleaning procedures! The study looked at two different police districts in Norway, and an important thing to note was that there was no standard universal procedure regarding washing/cleaning up after examining exhibits or evidence taken from crime scenes. The one thing they had in common was washing examination tables and floors, but there was no specific cleaning attention paid to regularly touched spots. In one district, examinations involving blood stains were followed with a cleaning regime of Rely+On Vircon; otherwise, household soap was used. In contrast, ethanol wipes were the chosen implement of

sterilization of the second district. Crime scene equipment maintenance was left to the officer’s discretion (e.g., washing when visibly stained).
Environmental DNA levels were gauged by swabbing or mini-tape on surfaces or objects touched during investigations such as examination rooms, equipment in cars, and photographic equipment. Samples were also taken from dirty gloves “directly after touching the questioned areas” and while they remained in the officer’s possession. Surfaces’ risk of contamination was categorized as either low (areas where a transfer is unlikely, e.g., not normally touched or some breach of protocol), medium (areas likely to be touched in the duration of an examination), or high (areas or objects directly in contact with the evidence).
Police-staff contamination control
However, it is not just cross-contamination between exhibits. It is also the real world possibility of officers or others handling the evidence contaminating evidence. To control for this, reference profiles from 51 officers (mainly CSI) and 84 NIPH staff were compared to DNA profiles from the 3,545 cases (or a total of 25,500 samples) they worked on or had access to. They used LR mix (forensim) software, fixing drop out probability to 0.01. “Matches with likelihood ratios above 10,000 were further examined manually based on allele matches and peak heights with an allowance of 4 allele dropouts.”
Negative controls
The question then becomes, Can DNA be “inadvertently transferred to negative controls during the evidence examination process, even if stringent conditions are enforced?” During an investigation, there are no regular restrictions for handling (like preventing bare hands from touching evidence bags) or storing and transferring evidence bags.
In order to test for potential handling contamination, negative controls were needed to see if they will test positive for DNA under the aforementioned conditions. In a clean environment (pre-cleaned with the power of RNase AWAYTM) and preparers suitably clad in full protective clothing, thirty pairs of sterile swabs and twenty autoclaved, irradiated strips of cloth were each placed into their own evidence bag. The bags were then handled by bare hands to deposit DNA.
Sample processing
In the laboratory, retrieval of the negative controls from their bags was designed to closely mimic casework activity. Swabs were either pulled out by a gloved hand or shaken out, after which, the tips were cut off and relocated to an extraction tube. Strips of cloth were removed by gravity and slightly tilting the bag, then using mini-tape to sample each side. The mini-tape was then cut up and also relocated to its own extraction tube. Cotton swabs also sampled the outside of the bags containing the negative controls as well as 17 actual case-related evidence bags.
Sample extraction was carried out with 5% Chelex solution (200 microliters for swabs and 250 microliters for mini-tape) and quantified with Quantifiler Duo Kit on the 7500 Real-Time PCR system. Amplification used the PowerPlex ESX 17 Fast System kit (Promega) and GeneAmp PCR System 9700 (for lower than optimal concentrations). Applied Biosystems 3500xl Genetic Analyzer processed the samples, and the results used GeneMapper ID-X Software with a limit of detection of 200 RFU.

Discussions

Regarding DNA profile comparisons, there were 11 cases of domestic contamination (from officers working on the case) and six cases of foreign contamination (from officers not involved in any way).

In light of police procedure of handling and examining evidence before DNA analysis, it is speculated that this contamination could be due to environmental DNA via multiple transfers from gloves to exhibits. In contrast, domestic contamination could arise from inadequate protective equipment such as infrequently changed gloves or even indirect transfer from the outside of evidence bags (rare at 0.0024% vs contamination by police at 0.04%). New profiling systems may have increased the detection rate of contamination events, and police protocols fall short of matching the new standards required. Even though police contamination events increased, NIPH contamination has been on the decline with the main risk coming from before the bag even sets an anthropomorphized foot in the

lab.
This study undershot contamination rates both through procedure and interpretation. Using a relatively high extraction volume, they were able to obtain fewer detection events than their more concentrated counterparts. In analysis, detectable events were defined by likelihood ratios exceeding 10,000.
In previous studies, individuals have a different propensity for ‘shedding’ their DNA. Transfer probability depends “shedder status” (pg 128) as well as material and contact type. Items with a larger surface area have a higher probability of contamination because of the “higher possibility that cells from the outside bag can inadvertently come into contact with the fabric compared to the swab.” Also, the negative controls where contamination occurred had a higher degree of DNA on the outside of the bag with three different mechanisms proposed by the authors: either it was shaken onto negative controls during removal, said removal caused it to come into contact with the opening edges, or it may have been transferred to extraction tubes.
Although rare, contamination is a significant event when it does occur. It complicates mixtures and potentially excludes suspects; the question then becomes how feasible is it for police departments to adopt more stringent sampling and cleaning policies. Two such policy changes were already made to police protocol after this study came out: ditching substandard cleaning practices and switching to better detergents like DNA AWAY as well as cleaning regularly touched spots. Other suggestions include separating exhibits from victim and suspect, frequent glove changing, and implementing a national elimination database.