Screening and detection
As noted above, postmortem toxicological analysis usually starts with a drug screen. Certainly, a drug screen can never be a single test, and most commonly is an open-ended panel of tests designed to detect the maximum number of substances of toxicological interest. This approach has often been called a search for the ‘general unknown’. Careful distinction must be made between this open-ended approach and a panel or targeted approach, in which the testing protocol only detects specific substances or classes of substances (e.g. drugs of abuse). Such an approach should not be referred to as a drug screen because this misleads the reader of a report into believing that a broader range of substances has been tested for than can possibly be the case. Targeted testing is sometimes justified where the case history strongly indicates that a specific substance is involved, particularly where that substance is not detected by the methods usually employed in the ‘general unknown’ approach. However, most experienced toxicologists have encountered instances in which the suspected drug was not found, with an entirely different substance detected in a clearly fatal amount. There can be several reasons for this. It may be that the person has consumed some other person’s medication. Or it may be that the medications reportedly found at the scene were not those taken. In suicidal deaths, it is not uncommon for the victim or the family to try to hide the evidence of overdose.
In most forensic laboratories a drug screen consists of a panel of immunoassay tests and headspace analysis for alcohol and other volatile substances, combined with one or more broad based GC or high-performance liquid chromatography (HPLC) procedures, and sometimes TLC and other techniques. The GC screening tests frequently use a nitrogen–phosphorus detector (NPD), since the vast majority of drugs contain nitrogen, and therefore give a response in the detector, whereas non nitrogenous compounds, such as fatty acids, cholesterol and other lipids, do not. Similarly, most HPLC systems use ultraviolet (UV) or diode array detectors, since the vast majority of drugs absorb light in the region between 210 and 350 nm. However, increasingly, as technology improves and prices decrease, MS is replacing NPDs and electron-capture detectors in GC and in HPLC systems. Virtually all these approaches require some form of specimen preparation and chromatographic techniques such as TLC, GC and HPLC typically require sample extraction.
