Further validation of a multiplex STR system for use in routine forensic identity testing.

A polymerase chain reaction- (PCR) based short tandem repeat (STR) system has recently been developed for use in routine forensic identity testing. The methodology involves the simultaneous amplification of alleles at four loci on different chromosomes, followed by the fluorescent detection of products using an automated DNA sequencer. The adoption of this technology into operational casework offers several advantages over systems currently in use, particularly the ability to obtain results from very old or small samples, reduced operator time when compared with conventional DNA (single locus probe) analysis and the potential for automation. Validation studies were incorporated into the development work on this system. The scope of these studies has been extended by further investigation carried out in this laboratory to test the reliability of the system under normal operational procedures. It was demonstrated that the precision of size determination was sufficient for the discrimination of alleles and size windows for allelic designation were established. A collaborative exercise carried out in conjunction with two independent laboratories demonstrated the robustness of allelic designation. Having tested both the DNA quantification and amplification techniques against DNA samples from a wide range of animal and microbial species, it was confirmed that results are only obtained from higher primate DNA. The PCR methodology was tested with both simulated and real casework samples (over 250 in total). Reportable results were obtained from most items yielding extracted DNA. Approximately 20% of the casework items from which no grouping (ABO, PGM) nor SLP results were obtained, gave reportable STR results. A method for the routine purification of DNA extracts which failed to amplify was established and validated for use in forensic casework. The STR multiplex system developed by Kimpton et al. proved robust and reliable when tested under the operational procedures in place in this laboratory.