In part 1 of the article, we looked at some of the more common non-standard samples used to obtain DNA from an individual for DNA testing purposes. In this article we look at DNA extraction success rates from the various types of samples and how these can vary from one sample to another.

Non standard samples do not always guarantee that a DNA Profile will be obtained. Whilst for buccal swabs it is possible not to obtain DNA, such as in cases where the swabbing was not adequately performed or else the swabs grows mould due to damp storage conditions, in general problems with this type of sample are normally less than 2%. Therefore depending on the nature of the sample, there is usually associated a High, Medium or Low probability of success in extracting DNA from each type of sample (in some cases a percentage probability of success is provided).

High (80%) : For samples such as fresh blood stains or hairs with root where, for a qualified and experienced DNA laboratory, extracting DNA is considered relatively straightforward. In such cases, as long as the sample is handled properly both during collection and delivery to the laboratory, one can expect a high probability of success in extracting DNA.

Medium (50%-60%) : For samples such as a toothbrush or cigarette butts the success rate is influenced by a number of variables including heavy use vs light use and/or storage conditions etc. Therefore a heavily smoked cigarette butt found in an ashtray at home is more likely to result in a successful DNA extraction than a lightly smoked cigarette that has been thrown on the street and exposed to the elements (e.g. rain).

Low (20% - 30%) For samples such as teeth or bone, where extracting DNA is a relatively difficult and time consuming process, these normally need to be handled by a highly specialised laboratory. In such cases, the individual should consult the laboratory first to check their level of expertise, if they will accept the sample and the cost.

The classification above is however only a guideline, since as we have seen it is not just the type of sample that is important but also other variables can have an influence such as :

1. Poor storage conditions (e.g. damp environment, extreme heat etc)

2. Poor sample handling (possible contamination with other DNA or dirt by touching the sample)

3. Not enough DNA present (e.g. heavily used toothbrush vs used once only)

It is also important to note that the success rate will also depend heavily on the ability of the laboratory performing the test. Not all laboratories that can perform DNA Paternity testing on normal swabs are capable of obtaining DNA from a wide variety of other media. The more difficult the media (e.g. teeth and bone), the greater the specialisation and experience required to increase the chance of a successful DNA extraction.

Finally, it is also important to note that with non-standard samples there is always a risk that the extraction will not work. Therefore any person considering submitting such a sample, especially where it may be only a limited sample available (e.g. person is dead), should take this into consideration. Selection of an experienced laboratory (possibly of forensic specialization) will help minimize this risk.