The double-swab technique versus single swabs for human DNA recovery from various surfaces

Most crime scene DNA evidence is retrieved using cotton swabs. Since the late 90's, the double-swab technique has been favoured by many practitioners throughout the world. However, the superiority of double-swabbing over applying single wet swabs has not been broadly verified. Here we set out to evaluate the need for the second dry swab for various surfaces, aiming at mimicking the range of surfaces encountered at crime scenes: flat and ridged, absorbing and non-absorbing. For the tested non-absorbing surfaces, i.e., window glass, steel, brass, synthetic leather and ridged plastic, the first wet swabs gave at least 16 times higher DNA yields compared to the second dry swabs. In addition, second wet swabs gave more DNA than second dry ones, opposing the common notion that the purpose of the second swab is to absorb excess liquid. When ten experienced staff members sampled saliva stains on a window glass surface the variation between persons was considerable, with mean DNA yields for the first wet swabs ranging from 0.045 ± 0.022 to 0.13 ± 0.024 ng/μL. The first wet swabs gave 4-162 times more DNA than the second dry swabs, with higher DNA amounts on second swabs coinciding with lower amounts for first swabs. We show that for non-absorbing surfaces, the first wet swab takes up most of the cells in dried stains, making it less valuable to apply a second dry swab. The differences in DNA recovery between first and second swabs were notable also for absorbing surfaces. Double-swabbing may be preferable for some complex surfaces, but focusing on efficient sampling technique with single wet swabs is likely a better general approach.