Nationwide study on forensic genetic analyses in criminal cases in Denmark

Historically, forensic genetics research has focused on increasing sensitivity of DNA analyses, improving mixture profile deconvolution, and advancing forensic DNA intelligence methodologies. The aim of this study was to quantify the relevance of these areas using empirical data from forensic genetic casework in Danish criminal cases. We present a retrospective analysis, primarily covering the years 2016-2022, with additional data from 2023 to 2024. During the entire study period, we received an average of 36,605 samples per year of which 66 % were from high-volume crime cases and 34 % were from violent crime cases. In 2016-2022, 50 % of the samples were deemed unsuitable for standard PCR-CE analyses due to insufficient DNA quantity or quality. Among the reported profiles, 85 % were full profiles with alleles in all analysed loci. A match to a person of interest, a victim, or a profile in the Danish National DNA Database was obtained for 40 % of reported DNA profiles. In 2023, our input DNA threshold for PCR-CE analyses was lowered from 200 pg to 100 pg. This increase in sensitivity led to an immediate increase in the relative number of reported profiles from 50 % to 59 %, which underlines the importance of further optimization of laboratory protocols. Across the full study period, approximately two-thirds of profiles were mixtures, emphasizing the need for continuous research in mixture interpretation and the exploration of alternative methods to deconvolute sample information. Additionally, nearly 8000 single-source profiles from violent crime cases that did not lead to a match with any known individuals, were identified as potential candidates for supplementary analyses. This amounts to approximately 850 samples per year or 2 % of all processed samples that may be analysed with the purpose of obtaining investigative leads through forensic DNA intelligence including prediction of physical traits and age, biogeographic ancestry inference, familial searching or Forensic Investigative Genetic Genealogy.