Transfer of biological stains from different surfaces.

Highly sensitive short tandem repeat-based stain analysis of weak biological stains has been improved during the last years. The risk of transfer of cellular material from handled items contaminated with biological stains such as blood and saliva is of forensic relevance. Although the policemen working with crime scene items are very careful, there exists a potential risk of transfer contamination. To obtain estimates for the risk of stain transfer by handling, we have carried out an experimental study on 288 dried blood and saliva stains in two laboratories. DNA quantification showed only small amounts of DNA that could be transferred, especially from stains on paper and cotton. The saliva and especially the blood stains from plastic surface resulted in higher amounts of transferred DNA, depending on the relation of blood volume to included area: Of 192 direct transfers, 17% gave extracts above 10 pg DNA/μl, and only 3% of 96 secondary transfers resulted in amounts above 10 pg DNA/μl.