From dactyloscopy to the Minimum Surface Requirement (MSR): a metric to assess and obtain suitable human STR profiles

Friction skin ridge (Dactyloscopy) and DNA analyses have both played an essential role in forensic investigations for decades. The simultaneous and successful collection of a latent fingermark ridge pattern and the respective isolation of its human DNA can be a challenging task at a crime scene. Maximizing the quantity and quality of genetic information from this type of physical evidence has become one of the objectives of forensic science. Often, friction ridge impressions may lack discriminatory power due to low visual clarity of ridges and/or insufficient pattern area. In such cases, an optimized human DNA collection and extraction becomes crucial for obtaining a genetic identity from an unsuitable fingermark. Recent studies have proven that complete Short Tandem Repeat (STR) profiles can be acquired from touch DNA specimens. Although the impact of fingermark enhancement techniques on DNA recovery has been explored, no research has yet investigated the potential quantity of DNA that could be isolated per area of a ridge pattern. Here, DNA collection and extraction methods to determine the Minimum ridge Surface Requirement (MSR) to extract suitable human STR genotypes have been studied and optimized. Twelve participants, including biological males and females, provided ink prints of their index, middle, and ring fingers from both hands to calculate an average fingertip size to create a "standard" area for analysis across all donors. A standardized cutout template was created to fit every participant's latent fingermarks on pre-cleaned glass surfaces. Two fingermark "recharging-deposition-collecting" methods (i.e., Original Experiment and Optimized Experiment) and two swab types for DNA collection were compared (i.e., cotton swabs from Delta Lab (Spain) and 4N6FLOQSwabs® from Copan (Italy)). Then, ten participants deposited fingermarks for DNA extraction, quantification, STR amplification, and capillary electrophoresis (CE) for STR genotyping. Results showed that flocked swabs were more effective than cotton swabs at collecting DNA and produced suitable STR profiles with the optimized collection method. This research establishes the first MSR surfaces from which meaningful STR profiles can be acquired. These data provide key knowledge that assists crime investigators in prioritizing dactyloscopy, genotyping, or both analyses concurrently when examining latent fingermarks.