Not all DNA swabs are equal—but that may have more to do with the hand guiding them than the material at the tip, according to a new series of tests by scientists at University College London’s Centre for the Forensic Sciences.

The scientists tested both cotton and nylon-flocked swabs in pulling trace DNA from common non-porous materials: plastic-handled knives, plastic piping like that from pipe bombs, metal cable used in poaching snares, firearm metal, and glass, they report in a genetics supplement to the journal Forensic Science International.

It was the swabs used by an experienced scientist that were consistently better.

“In summary, cotton swabs can be as efficient at recovering trace DNA as nylon-flocked swabs, but the rate of recovery appears to depend on practitioner experience and/or the substrate type,” they write. “This, along with the variable recovery efficiency of mini-tapes, is being investigated further.”

The nylon-flocked swabs (COPAN FLOQSwabs) yielded efficiency of roughly 85 percent from the control seeded directly with DNA solution, while the cotton swabs (SceneSafe) yielded about 55 percent, they write.

However, the disparity disappeared with application on non-porous surfaces. Roughly 55 percent of the DNA was recovered from plastic knife handles.

Cotton showed several better efficiency rates on plastic piping, firearm metal, and glass, they report. But this was still just a fraction of the DNA that had been planted there (all less than 20 percent), they add. 

The metal cable similarly defeated both kinds of swabs, with negligible genetic material picked up (less than 2 percent efficiency).

But the value of experience may be proven in the data, they also add. An experienced forensic scientist worked the knife handles, with the higher recovery rate, while newly-trained analysts worked with the other materials, they add.

“This could suggest that practitioner experience may impact the efficiency of DNA recovery and is thus being investigated further,” they write.

Mini-tapes made by the British company WA were also tested for pulling DNA—but efficiency rates were all less than 17 percent.

Some of the same UCL scientists published a study in the same journal in July which looked at the secondary transfer of DNA from a handshake to a knife handle—a study that mirrors some previous work done by scientists at the University of Indianapolis two years ago, among others.