News April 12, 2021 8M

By Forensic Access

James Beard’s recent webinar on Understanding and Challenging DNA Evidence was very well received and we simply didn’t have enough time to answer all the questions that came-up in chat. So we decided to put the unanswered questions to James and other scientists in our biology team for an illuminating DNA Q&A.


What are the limitations of Streamlined Forensic Reports (SFR) and can they be relied upon by defence without further investigation?

The limitations of SFRs are simple, but also critical. They were designed to streamline the investigative and charging processes, they were not designed for use in court. With regards specifically to DNA, the SFR may just be a DNA match report and therefore in many respects it is extremely limited in its usefulness.

The key factor to consider is context. Ask yourself these questions: what does this document represent and how does it fit with my clients defence? What item(s) are detailed in the report? What does the match actually state? Is there a statistical figure applied? Is the context of how the DNA was deposited relevant? 

We do not expect you to digest and understand every detail contained within the report, after all that’s why we are here, but simply ensure you understand the basics of what it is you are being served with by the prosecution.

Whilst we would never say the prosecution are deliberately trying to withhold information, they may be doing so inadvertently as an SFR does not give scope for a full and thorough evaluation and documentation of the evidence. Forced by their format, they are extremely compartmentalised.

So, we would almost always recommend a full review into the work conducted, it could even be that this hasn’t actually been done by the prosecution yet either!


What features do you use to distinguish between primary and secondary DNA transfer?

The answer to this question could be the basis of a rather long thesis! But in very short format, the factors used in evaluating whether DNA on a surface has been deposited by primary transfer (i.e. ‘direct’ transfer) or secondary transfer (a two -step indirect transfer) include the following:


From the DNA result

  • The amount of DNA in the recovered sample
  • The nature of the DNA result (assessed by viewing the DNA profile itself)
  • The DNA process which has been used


From the sampled item

  • The type of surface the sample was recovered from
  • The size of the area sampled
  • The way in which the item sampled would be handled during normal use


From the case circumstances

  • The differing scenarios put forward by the parties involved
  • The ‘history’ of the item
  • The possible interactions which could feasibly occur
  • An assessment of environmental levels of DNA which may impact transfer issues


The list could go on. There is also a wealth of research which has been done to specifically try and deal with this question. The data this research provides can be very useful in developing our understanding of what might be expected given different scenarios, but there are always limitations in trying to apply data from structured experiments to real world case examples.

If during evaluations we reduce the question down to a very simple ‘Was the DNA deposited by primary or secondary transfer?’ then we minimise our chance of answering the question in a balanced and informative way. The best way is to expand the question to include as much information about the suggested scenario as possible. Once a DNA result has been interpreted to assess the source of any DNA, further evaluation of the result with respect to how the DNA has been deposited must always be done with the result set firmly in context.

Even then, sometimes we must just be comfortable with not being able to favour one method of deposition above another.


Crime Scene Investigators often say that once they have powdered an exhibit for fingerprints, and none are found, the exhibit is not suitable for DNA analysis due to contamination. Would you agree?

The item may still be suitable, but this will depend on various factors.  Once the item has been examined for fingerprints in non-DNA conditions it has been compromised. It has had a substance added to it and it has also been examined in non-DNA clean circumstances.  We therefore have to consider the limitations of any further analysis within the context of the case.

In considering the ability to actually obtain a DNA result, the adding of powder or other chemicals may reduce this by destroying DNA, but not necessarily, and it may still be possible to obtain a profile.  This will depend on the exact fingerprint treatments used so finding out exactly what has been done is useful for us to then evaluate the possibility of obtaining a result subsequently. 

In considering the issues of contamination (detecting DNA deposited by the examination rather than the investigated incident), we have to undertake any subsequent DNA examinations with the acceptance that the item may well have been contaminated: by the brush or other equipment used to treat it, by the people carrying out the examination and by the environment it has been examined in.  Staff contamination is perhaps less of an issue as whilst this may complicate a result or prevent a useful result being obtained, it should not falsely implicate an individual; provided the staff members DNA profile is known to us.

The greater concern is contamination from the environment or tools used that could transfer DNA from a different item (or case) to the item in question.  Whether this precludes DNA analysis very much depends on the circumstances of the case, previous results from the item and whose DNA may be of interest.  If, for example: the item has been screened for fingerprints in the same lab as items from a person of interest, then finding any DNA matching them could be contamination.  However, if the DNA of the person can be shown to reasonably not be expected to be within that environment then it may still be useful to proceed. 

Whether to proceed may also be influenced by the type of body fluid or cellular material we are looking for. For example: if visible blood staining is present then we may be able to obtain a DNA profile that we are confident is from blood, thus making contamination far less likely.

Overall, if such examination may be of use, I would recommend calling us to discuss.  The feasibility and limitations can be determined on a case by case basis.


What are pre-PACE barcodes and how do they impact SFRs?

Pre-PACE barcodes have the prefix 95XXXXXX and below, whilst PACE samples can either have the pre-fix 96XXXXXX or 3XXXXXXX. Essentially a reference sample which starts with 95 or below should not be used for evidential purposes. However, they can still be requested from the National DNA Database for intelligence only comparisons by the DNA reporting scientist. It’s important to remember that not all SFRs are written by a DNA reporting scientist, so if you have an SFR citing a pre-PACE barcode this must be referred back to the originator.


Why do experts so frequently combine swabs before examining?

Scientists have a number of reasons for combining samples. The main reason is typically to maximise the possibility of obtaining a DNA profile where there may only be small amounts of DNA present. While the chances of detecting DNA are maximised there is a balance between this and the increased possibility of obtaining mixed DNA profiles and limitations as to being able to determine which area the resulting DNA profile was obtained from.


How can solicitors determine whether an SFR which doesn’t provide a match probability is one that is capable of statistical evaluation?

The simple answer is that you can’t and it must be reviewed by a DNA reporting scientist.

Some SFRs may have had a prosecution DNA scientist write them, but they simply haven’t done the statistical evaluation yet because the police force hasn’t requested it (i.e. paid for it to be undertaken).

Others could have been written by a support staff member for the police force, confirming that the National DNA Database have identified a match between a crime scene item and a reference sample. These latter reports will not have been reviewed by any reporting scientist, and therefore they require appropriate review immediately.

Either way, we can help determine this for you, guide you in the nuances of the submission processes and advise you on the best course of action.


Clearly it’s essential to have context on the location and source of an item of evidence, so what level of detail do you receive from Crime Scene Investigators?

The amount of information received can vary considerably.  The scientist carrying out the original examinations for the police should be able to provide information as to the location of the item and case circumstances from the reports of the attending officer and crime scene investigator. They should also have an understanding of why the force want the item examined and any accounts of those involved. In cases involving blood, scene photography, medical reports etc may also be of use.  In sexual assault cases medical examiner reports and accounts of those involved can be crucial. 

It can sometimes be difficult for the prosecution scientist to obtain this information (depending on police forces, contracts, whether a defendant has made an account, and particularly where items have previously been processed in police laboratories), but ideally they would be provided with this information.  The CSI report is usually available to the prosecution scientist as this is made at the start of the case and often submitted to the lab with the police request.

When carrying out a defence review we often have an advantage of more information being available by that stage and may have access to later accounts by defendants or results of other examinations etc.  We would require all of the information discussed above to carry out a full review. 

Looking specifically at CSI reports: these usually detail exactly what has been examined, sampled and retrieved and are usually accompanied by photographs of the stain prior to swabbing or the item in situ at the scene. These can be very useful in determining exactly where was sampled, for example: seeing a blood stain in situ before it was swabbed may allow us to make comment as to how it was deposited originally. Similarly seeing the item in situ may assist with queries around indirect transfer of DNA. 


What is the threshold used to identify a ‘true allele’ and how can you confidently differentiate the ‘true allele’ from noise and artefacts in low DNA profile?

What a great question: understanding these thresholds, which allow us to decide when a peak within a DNA result is actually DNA, is vital in interpreting DNA profiles.

Every laboratory which sets up a DNA processing line has to go through a validation process. This involves testing lots of samples with a known DNA content and having a good look at the results obtained. It is during this process that the laboratory will discern what the various interpretation guidelines are, and set the thresholds which the reporting scientists will use to aid them in interpreting DNA profiles.

Therefore each laboratory will have a set of ‘reporting guidelines’ specific to them, and these guidelines will help scientists have confidence that what they are seeing is, as you say, a ‘true allele’ or in other words DNA from a contributing individual. So these thresholds will actually be different for each laboratory, and even within one laboratory they will constantly be reassessed and modified where necessary.


There were initial concerns raised surrounding STR and other type analysis on the basis that the programming and algorithms supporting the statistical analysis had not been released. Have they been resolved and is there any continuing ‘unease’ about the use of statistical evaluation for mixed DNA on that basis?

We would not have any particular concern relating to the algorithms used by these programs. They have been published, peer reviewed, validated and accredited by UKAS and some are being used and accepted by the courts worldwide. The amount of industry debate surrounding the methods used is such now that our concerns around the statistical methods used are greatly reduced.

However, what is still of concern is that interpreting these results is not just reliant on the algorithms used but equally on the validity of the assumptions made when setting hypothesis. We regularly observe the setting of inappropriate hypothesis, particularly around the number of contributors as new information comes to lights during the defence evaluation. Therefore reviewing these results can still be critically important.


What is the margin of error when using SGM, SGM+ and DNA17?

The profiling test itself is run with set controls.  If the controls pass, then any clear DNA components detected that meet the reporting criteria are designated as a particular component with no margin of error.  This would apply to SGM, SGM+ and DNA17.  So determining the actual clear components detected in a result is robust with no error margin.

SGM+ looked at 11 areas of a person’s DNA.  DNA17 looks at those 11, plus 6 more.  Therefore, DNA17 is a more discriminating technique. If an SGM+ profile was obtained from a sample and that matched a reference profile at all 11 areas a statistic of over a billion would be quoted.  If DNA17 profiling of the sample and reference showed a difference at one or more of the extra areas, then that person would then be excluded as having left the DNA.  This does not mean the SGM+ result was an error, it is just that we now have more information. 

In reality, if a person matches at all 11 SGM+ areas of a non-mixed straightforward full DNA profile then it is highly unlikely that DNA17 profiling would then eliminate them. It is far more likely to further confirm the match.  The exceptions to this would be when dealing with complex mixed DNA results, or any possibility of related individuals.  In these cases, I would always recommend a DNA17 reference sample be obtained to allow a fuller comparison. 


Why, when the full DNA components of a father and son are found at a crime scene, is a statistical analysis not always possible? And could either be excluded as a contributor?

Statistical analysis calculate the likelihood of obtaining matching DNA profiles if components match by chance. However, when there are related individuals, they will have components matching due to inheritance as well as by chance and the analysis is therefore not valid.

Where full DNA profiles are obtained it would almost always be possible to distinguish between the profiles of a father and son and so exclude one from having contributed. This would become more complex where a mixed DNA profile has been obtained but, depending on the result it may well still be possible to exclude on and/or the other.


Is there a time limit for challenging a SFR?

Yes and it is taken directly from the SFR:

‘If the DNA evidence is not accepted by the defence, then this SFR1 must be challenged, either on the basis that you dispute the DNA match, or another reason, for example you dispute the mechanism of transfer of DNA. In accordance with Crim.PR 19.3(2), the defence is required to serve a response to this report as soon as practicable, and in any event not more than 14 days after service of the SFR1.’

Whether this timeframe is routinely adhered to, or enforced, is open to discussion.


When will it be possible to age someone from their DNA?

It would be wonderful to be able to age people by their DNA but I don’t see that option becoming available to us anytime soon. While there are limited changes to our DNA as we age, none of these are tested using the methods available to forensic scientists and it may be that they we are never able to accurately predict a person’s age from  their DNA, even if these changes were detected.


Is there a test for vaginal fluids?

In short the answer is ‘no’. There have been some attempts in the past to identify vaginal material, such as by the use of ‘Lugol’s Iodine’ test or by assessing the characteristics of the nucleus within the cells found in vaginal secretions, but these have been shown to be unreliable and have not been used for some time.

There is no accredited test currently in use within forensic science providers in the UK. Research is always ongoing looking at new ways of identifying body fluids, and some of the work with ‘messenger RNA’ (mRNA), a component of a cell’s genetic processes does looks promising, so who knows what might be available in years to come?

To find out more about the forensic services we offer, fill-in our online contact form or Tel: 01235 774870 to speak with our team.

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