Evaluation of DNA Loss in Touch Samples from Various Substrates

Touch samples, by nature, may contain a low quantity of DNA, and the manipulations of collection and analysis can further reduce this amount. Studies have tracked up to a 70-90% loss of DNA during the performance of standard protocols. It has not been clear, however, at which point(s) during collection and/or analysis the DNA is primarily lost. To evaluate loss during collection and extraction, three mock fingerprint controls were developed. Twenty microliter mock fingerprints (0-1500 cells, 0.00 – 9.00 ng DNA) were pipetted: 1) On to a surface to measure the quantity of material lost to the entire collection-extraction process, 2) On to the swab to determine the amount of material retained on the surface and swab after extraction through minor calculations, and 3) Directly into the extraction tube to monitor the material lost due to the extraction procedures. Five-point standard curves were constructed by plotting DNA recovered after extraction vs. initial DNA added for the mock fingerprints. Validation of the mock fingerprint controls was done first on glass slides to serve as a baseline followed by three crime scene objects: steering wheel, brass door plate, and glassine (drug) baggie. The R-squared values for the curves from all four substrates ranged from 0.9890-0.9999, validating the use of mock fingerprints as positive controls. The results demonstrated an average total percent loss of 67% was found to occur from collection to extraction: 10% remained on the surface, 24% was retained on the swab, and 33% was lost to the extraction. By tracking the loss and looking at the effects surface type may have on touch DNA yields, this will improve the recovery of these samples and increase the use of touch DNA in forensic science.