Exploring the limits for the survival of DNA in blood stains

It is generally recognized that usable DNA may be retained in dry biological stains for years. We have explored the environmental limits for this property. Air-dried blood stains were incubated at different conditions of relative humidity (RH) and temperature. The quality of the extracted DNA was assessed by the ability to amplify 273 bp and 1600 bp DNA fragments by PCR, and by quantitative estimation of a 147 bp DNA fragment using real time PCR. Despite the fact that the availability of water is important for processes that degrade DNA, no significant difference was observed in the stability of DNA at 50%, 80% or 93% RH at room temperature or at 35 °C, and even the 1600 bp fragment was amplifiable after one year. Microbial growth was not observed at these conditions and the number of template molecules did not drop significantly over time. At 100% RH, however, microbial growth was observed after varying amounts of time. This may explain the decreased stability of DNA observed at these conditions. Even so, the 273 bp fragment was amplifiable for at least 3 months, and the 1600 bp fragment for at least two months. Microbial growth was not observed at higher temperatures (45-65 °C) at 100% RH, and the 1600 bp fragment was amplifiable after eight months at 45 °C, but only survived for one month at 55 °C or 65 °C. Thus DNA remains amplifiable in blood stains for many months, even at extreme RH and temperatures up to 45 °C. Even in humid climates the average RH is usually not more than 80% and RH rarely exceeds 93%; therefore we conclude that normal climatic conditions are not critical for the long time survival of DNA in untreated blood stains.