Swabbing is a universal technique to recover DNA in any forensic investigation. Sometimes, though, the surface is not right for swabbing, or the area is too large. So, what then?
In a recent study published in the Journal of Forensic Sciences, researchers with the FBI recommend a wet-vacuum alternative—the M-Vac system. The researchers say swabbing should remain the preferred collection method on substrates with visible stains and/or nonporous surfaces; but, the M-Vac wet-vacuuming system should be considered an alternative collection method, especially on difficult porous surfaces.
Jared Bradley, the CEO of M-Vac Systems, Inc., once described the technology to Forensic as “a mini-hurricane of sterile solution combined with vacuum pressure.” The M-Vac aggressively sprays a sterile solution onto a surface and simultaneously applies vacuum pressure to collect the solution and whatever DNA material is present on the surface. The solution and DNA material are then run through a filter or a microcentrifuge. The tool was originally developed by the late Bruce Bradley, a microbial scientist, to identify food contamination, but it proved to be too sensitive in finding the most miniscule traces of potential pathogens—a concern not applicable forensic DNA analyses.
In the study, the researchers compared traditional swabbing with the M-Vac in three different ways. First, DNA recovery from items collected with the M-Vac was compared with DNA recovery using a conventional wet‐swabbing and an automated magnetic bead‐based extraction technique. Of the 20 porous substrates, the M-Vac yielded consistently greater nDNA yields than the wet‐swab method on all but two surfaces—cinderblock and unpainted drywall. Additionally, the amount of DNA recovered with the M-Vac was several‐fold greater—the vacuum system yielded an average of 12 times more nDNA and 17x greater mtDNA.
Second, the researchers used the M-Vac on 10 previously swabbed substrates to potentially recover any uncollected DNA. The M-Vac recovered additional DNA on 9 out of 10 substrates, the only exception being satin-painted drywall. At minimum, the M-Vac recovered at least as much as swabbing, bit at maximum, the vacuum system recovered 46x more.
“Altogether, wet‐vacuuming after swabbing yielded an average of 10 times more nDNA and nine times more mtDNA as compared to the initial wet‐swabbing. These results demonstrated that considerable DNA remained in or on these substrates after wet‐swab collection,” the researchers write in their paper.
Lastly, efficiency of the collection techniques was assessed by using the same downstream extraction method for both the M-Vac and wet‐swab collections. The M-Vac yielded a modest increase in DNA yields, but not enough to be considered statistically significant for the purposes of the study.
Overall, the study demonstrated that the M-Vac system is capable of collecting diluted blood on multiple types of challenging surfaces—especially difficult or porous surfaces—often with increased collection efficiency over traditional swabbing techniques.
But, the researchers still recommend traditional swabbing as the go-to as it is “more convenient, simple, time‐efficient and inexpensive relative to the wet‐vacuum method, and can be very effective in collection from visible stains or areas of repeated handling/contact.”
“Other than both devices collect DNA material from the surface, there aren't many other similarities between the swab and the M-Vac,” Bradley recently told Forensic. “It’s kind of like comparing a hand broom to a carpet cleaner. One is a simple device that is effective in some cases, and the other is a much more robust machine that is used when it's obvious the broom won't cut it or has been tried and failed. Same with the M-Vac—it brings forces to bear on the bio-stain or touch DNA that the swab just wasn't designed for. So, the cost difference will also be obvious. Both have their place in the DNA collection toolbox. Our general rule is if the CSI or lab serologist can see the stain, they don't need the M-Vac. If not, then they may want to use the M-Vac to make sure they can collect whatever DNA material is there.”
According to the study, the estimated start‐up cost for the M-Vac ranges from $43,000 to $45,000, while the cost per sample is about $90, compared with less than $15 for the wet‐swab method.
That being said, what is the price tag for recovering previously undiscoverable DNA that leads to a case breakthrough?