Contamination

Question 1

Define contamination?

Answer 1

• to make something impure by exposure of or an addition of a polluting substance

Question 2

How does the new DNA17 multiplex make contamination easier?

Answer 2

• it is very sensitive so capable of detecting even smaller amounts of DNA
• improved sensitivity = contaminated DNA is more likely to be detected

Question 3

What is difference between contamination and adventitious transfer?

Answer 3

• contamination - applied to forensic process as transfer of foreign DNA to sample AFTER crime has been discovered (scene attendance through to sample collection/testing)
• adventitious transfer - transfer from innocent activity before crime event

Question 4

What is the issue with undetected contamination?

Answer 4

• complicates interpretation
• increases risk that result could is incorrectly attributed
• increases risk that result is given undue significance
• could diminish the value and reliability of the forensic evidence
• could change single source sample into a mixture
• could give a false positive result

Question 5

What are the two constituents that contamination could manifest as?

Answer 5

• major and minor

Question 6

What are the two types of contamination that it could be?

Answer 6

• one off - affecting single tube in batch of samples
• blanket/gross event - affecting whole batch of samples

Question 7

What are the 3 criteria that need to be fulfilled for contamination?

Answer 7

• needs to be:
• source of DNA available to be involved in transfer event
• opportunity for DNA transfer to occur
• mechanism by which DNA transfer is achieved

Question 8

What are the first potential sources of contamination?

Answer 8

• individuals involved in different roles of forensic process:
• police
• scenes of crime officers
• pathologists
• forensic scientists

Question 9

What are the second potential source of contamination?

How can this be understood and therefore avoided?

Answer 9

• consumables or reagents used in forensic process, packaging and environment in which test is carried out (including other forensic samples processed in lab environment)
• important to understand where our consumables are produced and the quality standards which are associated with their manufacture
• essential to have quality managed lab process with appropriately trained staff

Question 10

What are three contamination opportunities?

Answer 10

1 - from person to stain
2 - from consumable to stain
3 - from stain to stain

Question 11

What can be said about the mechanisms of transfer?

What are the three mechanisms for transfer?

Answer 11

• the mechanisms for transfer are provided by the opportunities and can be straightforward or more complicated

1 - direct primary contact
2 - secondary transfer
3 - tertiary

Question 12

What is an example of person to stain contamination?

Answer 12

• poor use of PPE (facemasks and hairnet) which should provide a barrier between operator and sample

Question 13

What is an example of reagent to stain contamination?

Answer 13

• direct transfer from DNA already present in a tube which is not sufficiently free of DNA for forensic purposes

Question 14

What can issues with lab processes cause?

Answer 14

• DNA transfer and contamination is facilitated inappropriately by cross-contamination between samples

Question 15

How could we prevent contamination transfer?

Answer 15

• training of staff to a recognised standard (knowledge of what could happen and where to look for it)
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• procurement of consumables from a source supported by quality standards

Question 16

What are detection methods for contamination?

Answer 16

• elimination databases for lab staff
• appropriate quality checks within forensic process

Question 17

At what processes can contamination occur?

Answer 17

• collection
• extraction
• amplification
• injection

Question 18

What type of DNA increases the risk of contamination?

Answer 18

• degraded or low template DNA

Question 19

What is allele drop-in

Answer 19

• a type of profile contamination where 1 or 2 alleles are present in DNA profile that are not inherent to DNA extract
• not reproducible on reamplification

Question 20

How could allele drop-ins occur?

Answer 20

• dust particles from lab environment
• plastic ware used within DNA process

Question 21

What causes differences in peak heights of repeated application of the same input to DNA?

Answer 21

• stochastic effects

Question 22

What is touch DNA?

Answer 22

• trace DNA
• DNA analysis at crime scene only requires 7-8 human skin cells (these can be transferred from just touching objects

Question 23

Why has touch DNA been criticised?

Answer 23

• criticised for false positives due to contamination
• in 2008, parents of murdered child (Jon Ramsey) were cleared after unknown male’s DNA was found in her underwear
• but this sample was so small that this DNA could have been from person who packaged underwear in factory

Question 24

Gareth Williams case

Answer 24

• person to sample contamination
• SOCO to exhibit
• subsequent testing produced a DNA profile which caused misleading investigation and wasted resource

Question 25

Phantom of Heilbronn

Answer 25

• Michelle Kieswetter
• consumable to sample contamination
• from 1993 to 2009 detectives matched DNA of serial killer in over 40 crimes in many countries
• in 2007 police officer Michelle was murdered in Germany and DNA matched to unknown serial killer
• cotton swabs used to collect DNA were not certified for human DNA collection and sterile (they were all from same factory)
• workers DNA on cotton swab before police received them and used them

Question 26

Manchester Rape

Answer 26

• sample to sample contamination
• failure in lab practise which involved reusing sampling equipment resulted in DNA profiles being given inappropriate relevance in context of case