The evaluation and implementation of match criteria for forensic analysis of DNA

This study describes a method for establishing match criteria used in forensic DNA typing. The validity of applying different match criteria based upon the molecular weight of a DNA band is discussed. The match criteria presented allow visually matching DNA patterns to be confirmed by computer assisted image analysis over the entire range of the sizing ladder. Approximately 5000 intragel and 5000 intergel comparisons were made between the restriction fragment length polymorphism (RFLP) DNA band sizes obtained from casework, mock cases, and environmentally insulted samples and the band sizes obtained from their corresponding bloodstain standards (controls). Analyses of these data suggested that fragments located in different molecular weight size regions of an analytical gel required different match criteria for assessing a visual match. The results of these analyses support the use of the following match criteria: Intragel 0.5-10 kb = +/- 1.7%, 10-15 kb = +/- 3.2%, 15-22.6 kb = +/- 5.8%; Intergel and blind control 0.5-10 kb = +/- 3.0%, 10-15 kb = +/- 4.2%, 15-22.6 kb = +/- 10.0%; and human cell-line K562 and the monomorphic locus D7Z2 = +/- 2.5%. Each match criterion was also evaluated with respect to the distance in millimeters between matching bands throughout the 0.5-22.6 kb molecular weight size range. Applying these match criteria to different gel regions has been shown to be valid and reliable in comparisons conducted on more than 10,000 validation samples, in over 500 forensic cases and in more than 200 searches of a criminal sexual offender (CSO) database containing over 5000 individuals.