Risk of Contamination Increases
The possibility of contamination of cold case samples cannot be over-stated. The older the case, the less likely that care was taken by investigators, crime scene personnel, or even laboratory analysts to avoid contamination. This is not an indictment of the people involved in the case – it is simply an unfortunate side-effect of the fact that DNA testing has evolved so rapidly. Items that would never have even been considered for DNA testing 10 years ago are now routinely processed in the lab. Prior to the advent of the Polymerase Chain Reaction (PCR) a bloodstain the size of a quarter needed to be available for DNA testing. Now, a bloodstain the size of a pin-head (or even less) is needed. And, it is not just blood, semen, saliva, or other body fluids that are being examined in the lab. Labs are also testing skin cells.
Although it might be possible to get a DNA profile from evidence that has already been processed by another section of the laboratory, one must be aware that techniques to avoid contamination might not have been in place at the time the evidence was collected. For example, while we now know that DNA profiles can be obtained from fingerprints – if a print was processed using powders and brushes that had been used on other items of evidence first, it is quite likely that the brushes and powders are contaminated with DNA. If the firearms section test-fired the murder weapon to perform comparisons without wearing gloves, the DNA profile that will be developed from the surface of the gun is probably going to match the firearms examiner. If the medical examiner fails to properly clean the fingernail clippers between bodies, artificial mixtures of DNA can be created. Even if the item was only processed by the DNA section to begin with, there could be contamination issues. Prior to the advent of PCR, different protocols and different levels of cleanliness were in place because it just wasn’t possible to get a DNA profile from a small amount of DNA.
Above, we discussed the transfer of DNA from the swab itself to the packaging and mentioned that DNA material may remain on the interior of the swab packaging even if no swab material is left. That is a good thing, in terms of possibly having additional material to work with. However, this same transfer can occur if items are packaged together. For example, if all the clothing items from a victim are packaged in the same large brown paper bag, it is possible for DNA to transfer to different areas on the same item of clothing as well as to different items of clothing in the same bag. This, in turn, can lead to problems interpreting the results or even reconstructing the crime.
Another potential avenue of contamination is if the case has already been to trial. It is possible that evidence items were opened and handled by attorneys or jury members thus leading to another possible contamination scenario.