Maximization of STR DNA typing success for touched objects

In order to produce database-eligible DNA profiles from touched objects, each testing procedure including sample recovery, extraction, amplification and separation was evaluated and optimized. The developed methodologies were tested on control samples as well on fingerprints deposited on a variety of substrates such as credit cards, keys, and pens. All samples were amplified in triplicate to confirm the presence of each allele and to detect drop-ins. Overall the modifications implemented produced reproducible results for DNA titrated to 20 pg. For DNA dilutions, 25 pg routinely resulted in full profiles, and 12.5 pg determined 76.9% of the database loci tested. Similarly, for the touched objects, 75.8% of the 20-pg to 100-pg samples yielded database-eligible profiles; the remaining samples either were mixtures or contained an insufficient number of allelic calls. Here, the three-amplification approach was crucial and produced more complete profiles with confidence in the allelic assignments. DNA amounts below 20 pg did show partial profiles with correct allelic determinations that could have been compared in a specific case but were often too incomplete for database entry.