Influence of Surface Material, Cleaning Frequency and Swab Type on Touch DNA Recovery from Entrance Door Handles: A Simulated Study

Touch DNA has become an increasingly valuable tool in forensic investigations, particularly in the absence of bodily fluids. However, its recovery is highly variable and influenced by multiple factors, including surface type, swabbing technique, and environmental conditions. Understanding how these variables interact is essential for optimizing evidence collection and interpreting complex DNA mixtures. This study systematically evaluated the effects of surface material (brass, stainless steel, plastic, wood), cleaning frequency (none, weekly, daily), swab type (IsoHelix® vs. rayon), and transfer mode (primary vs. secondary contact) on the quantity and composition of touch DNA recovered from door handles. A total of 240 samples were collected using a full factorial design. DNA was extracted, quantified, and profiled using standard forensic workflows, and statistical analyses were used to assess differences in yield and contributor dominance. IsoHelix® swabs consistently outperformed rayon swabs, recovering two to three times more DNA across all surfaces. Wood and plastic handles yielded significantly higher DNA quantities than metal handles, with brass showing the lowest recovery. Increased cleaning frequency substantially reduced DNA yield and elevated the presence of background or unknown contributors. Mixture analysis revealed that the last person to touch a handle was the major contributor in 74% of wood, 71% of plastic, 55% of stainless steel, and 49% of brass samples. Logistic regression confirmed surface material and cleaning regime as significant predictors of contributor dominance, while swab type had a stronger influence on DNA yield than on contributor attribution. Overall, touch DNA recovery is governed by a complex interplay of surface characteristics, sampling tools, and environmental history. While IsoHelix® swabs demonstrated superior performance for door handle sampling, their effectiveness may not generalize to all exhibit types. These findings highlight the importance of selecting context-appropriate swabbing methods, documenting surface hygiene history, and applying probabilistic frameworks when interpreting low-template or mixed DNA profiles. The study provides practical guidance for forensic casework and contributes to the refinement of trace DNA sampling strategies.