Methods of Analysis – Confirmatory Testing

In a forensic setting, truth must be established beyond any reasonable doubt in order to guarantee the rights of all parties. This obvious but nevertheless essential principle, when transferred to analytical toxicology, requires unequivocal identification of a xenobiotic in a sample. The identification of a drug (most commonly a xenobiotic) in a sample is quite often a critical step in a forensic investigation, proving the link between drug use and a judicially relevant event. These events could be drug dealing, driving under influence, a workplace accident, a drug-facilitated sexual assault, or even a homicide. In forensic toxicology, it is generally accepted that identification must be accomplished through the use of at least two analytical techniques based on different chemical and physical principles, supporting the results of each other. Therefore, whenever a presumptive identification is obtained, a second confirmatory test must be applied producing an analytical result as independent as possible from that of the first-level (screening) test. The rational behind this principle is that the chance that both tests (screening and confirmatory) may give a false positive identification must be as low as possible. Therefore, a confirmatory method based on a detection principle very similar to that of the screening test is unacceptable (e.g., confirmation of an immunochemical result with another immunochemical method).