Evaluation of successive DNA extractions from cotton and flocked swabs.

The use of swabs to collect biological samples is a current practice in forensic investigations. Once a sample is collected, the DNA is extracted and used for testing at the crime lab. DNA extraction can often be sub-optimal in yield, and identifying new approaches to obtain larger DNA quantities from casework samples is critical. The goal of this study was to investigate the success rate of obtaining sufficient DNA yields from previously extracted swabs. Reference and case-type samples were collected using different types of swabs and genomic DNA was extracted from samples multiple times using only the QIAamp DNA Investigator Kit or using this kit in combination with the Investigator Lyse&Spin Basket Kit. DNA samples were amplified using the GlobalFiler™ PCR Amplification Kit and separated on a SeqStudio Genetic Analyzer. All types of swabs and extraction methods yielded DNA after three extractions, and complete DNA profiles were produced for reference and case-type samples up to the third extraction. This provides a proof-of-concept that it is possible to recover DNA from previously extracted swabs in sufficient yield for full STR profiles. Also, trace DNA casework swabs can be extracted twice in succession and extracts pooled to increase yields for necessary DNA tests.