Effect of swabbing technique and duration on forensic DNA recovery

Various factors have been shown to affect performance of the conventional wet-dry double and single wet swabbing techniques to recover DNA, such as pressure and angle of application, volume and type of wetting agent, and swab type. However, casework laboratories in some jurisdictions have recently adopted different swabbing techniques that include wet-moist double swabbing and moist-dry single swabbing. Factors affecting the effectiveness of these recent techniques in maximising DNA recovery therefore need to be investigated. Here, the performance of traditional and recent swabbing techniques was compared and the impact of swabbing duration on DNA recovery was investigated. Ten µl aliquots of a known concentration of DNA extracted from human blood were deposited on pre-cleaned DNA-free cotton swatches (porous) and porcelain tiles (non-porous). Five swabbing techniques were used, of which three were double swabbing techniques: wet-moist, wet-wet and wet-dry, and two were single swabbing techniques: wet and moist-dry. For a ‘wet’ or ‘moist’ swab, 100 or 50 µL water was added, respectively. For a moist-dry swab, water was applied to one side of the swab, leaving the other side drier. Each swabbing technique was applied for two durations, 15 and 30 s per swab, with 5 reps of each combination (n = 100 plus controls). All samples were extracted and quantified, and a sub-set was profiled. The results showed that the wet-moist double swabbing technique with a swabbing duration of 30 s maximised DNA recovery from cotton. From tile, a single wet or moist-dry swab maximised DNA recovery, but increasing swabbing duration from 15 to 30 s had no impact. These data can be used to inform standardisation of DNA collection protocols across casework laboratories.