Managing Porous Substrate Wicking in Trace DNA Collection: Impact of Swab Wetting Volume on Fabric Evidence

Abstract

Trace DNA recovery from clothing remains highly variable and strongly dependent on pre-analytical sampling conditions. While moist swabbing is widely adopted for fabric substrates, the volume of liquid applied to the swab is rarely standardised and has not been systematically evaluated as a mechanistic determinant of recovery efficiency. This study investigated the effect of controlled swab wetting volume on trace DNA recovery and STR profile quality from worn fabric substrates with differing absorbency, and quantitatively linked biological outcomes to physical measurements of fabric absorbency and swab wetness loss.
Garments representing common forensic clothing materials (cotton, cotton/polyester blend, polyester, nylon, denim, and fleece) were worn by volunteers under routine daily activity conditions. Sampling was performed using cotton swabs moistened with 100, 150, or 200 uL of sterile distilled water under a standardised protocol. Fabric absorbency was quantified gravimetrically, and swab wetness loss during sampling was measured to estimate liquid sequestration into the substrate. DNA was extracted, quantified, and STR typed using a single analytical workflow. DNA yield, STR profile completeness, allele counts, and RFU values were evaluated using mixed-effects statistical models.
Increasing wetting volume significantly enhanced trace DNA recovery and STR profile completeness (p < 0.001), with the strongest effects observed for highly absorbent fabrics (cotton, denim, and fleece) and smaller effects for low-absorbency materials (nylon and polyester). Fabric absorbency and swab wetness loss were independently associated with DNA yield (p < 0.01), supporting a physical model in which textile substrates act as competing liquid sinks that reduce effective wet contact at low wetting volumes. Modest increases in DNA quantity produced disproportionate gains in full STR profile recovery, consistent with threshold behaviour in low-template amplification. No systematic PCR inhibition was observed.
These findings demonstrate that swab wetting volume is a critical, substrate-dependent parameter governing trace DNA recovery from fabrics. By linking textile liquid-transport behaviour with biological recovery outcomes, this study provides a mechanistic and predictive framework for fabric sampling and supports the transition from heuristic wet swabbing toward quantitatively controlled, substrate-aware collection protocols. Optimisation of wetting volume represents a simple, low-cost intervention capable of materially improving evidential yield from clothing evidence without changes to consumables or analytical workflows.