Contamination

Question 1

How is the risk of contamination increased using the DNA 17 Multiplex?

Answer 1

• The DNA 17 multiplex is very sensitive and is capable of detecting ever smaller amounts of DNA.
• The improved sensitivity of the new multiplex means that contaminating DNA is more likely to be detected.
• This means that understanding the mechanisms for contamination is an important part of a DNA 17 assessment.

Question 2

What is a one-off contamination event compared to a blanket contamination event?

Answer 2

• A one-off event is where the incident of contamination only affects a single tube in a batch of samples
• A blanket contamination event is where the whole batch of samples is contaminated.

Question 3

What are the three criteria for contamination to occur?

Answer 3

1. There needs to be a source of DNA available to be involved in the transfer event.
2. There needs to be an opportunity for DNA transfer to occur.
3. There needs to be a mechanism by which the DNA transfer is achieved.

Question 4

Who can the contaminating DNA come from?

Answer 4

• The first potential source of contamination concerns the individuals involved in the different roles of the forensic process such as:
  
  • Police
  • Crime Scene Investigators
  • Pathologists
  • Forensic Scientists etc.

Question 5

What is the second and final category of sources of DNA contamination?

Answer 5

• The second category is that of contaminating DNA originating from any consumables or reagent used in the Forensic Science process and the environment in which the test is carried out.
  
  • Must understand where our consumables are produced and the quality standards associated with manufacture
• The final source of contaminating DNA is other forensic samples which are processed concurrently.

Question 6

What are the different mechanisms of DNA transfer associated with each category?

Answer 6

• From person to stain: contamination may be due to poor use of PPE.
• Reagent to stain: The transfer may involve direct transfer from DNA already present in a tube which is not sufficiently free of DNA for forensic purposes.
• From stain to stain: there may be an issue with laboratory processes, so DNA transfer and contamination is facilitated inappropriately by cross-contamination between samples

Question 7

How are contamination events prevented or reduced?

Answer 7

• Forensic practitioners can introduce prevention/mitigation and detection tools for contamination.
• The processes and tools are designed to reduce opportunities and disrupt mechanisms for contamination.
• E.g. preventative measures may include training of staff to a recognised standard and procure reagents from a source supported by quality standards.
• Detection methods include introduction of elimination databases for lab staff etc.