The techniques used depends on the substance measured and on the toxicological problem.
Immunological methods based on antigen-antibody reactions are often used for screening of several different drugs of abuse in urine, hair or oral fluid. These methods can detect not only the mother substance (e.g. cocaine) but also the metabolite (e.g. benzoylecgonine). They can demonstrate groups of substances with similar chemical structures such as benzodiazepines. The strenght of the signal in a screening test does not necessarily correlate with the concentration of the analyte. Also a screening test can in some cases provide a false positive result e.g. in a subject who never consumed drugs. So a positive screening result has always to be confirmed. For the identification and exact quantification of a substance tested positive in a screening assay chromatographic techniques such as gas chromatography coupled to mass spectrometry (GC-MS) or liquid chromatography coupled to mass spectrometry (LC-MS/MS) have to be applied. The required reliability and the unequivocal identification of a substance in forensic toxicology can only be obtained with confirmatory chromatographic methods.