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Background: There is increasing interest in the use of oral fluid as the matrix for the detection of drugs of abuse which requires the use of sensitive immunoassays to achieve the low detection limits required. The use of liquid chromatography linked to tandem mass spectrometry (LC/MS/MS) is explored as a possible replacement for immunoassay in screening for drugs of abuse in oral fluid samples. Methods: Oral fluid samples collected from 72 subjects attending an addiction clinic were screened for opiates, cocaine, methadone and benzodiazepines using both enzyme-linked immunosorbent assays (ELISA) and LC/MS/MS. The latter analysis used a short gradient elution with individual drugs detected by multiple reaction monitoring using tandem mass spectrometry. Results between the two methods were compared qualitatively using the cut-off concentrations defined by the ELISA assays. Results: With regard to the ELISA assays which show group specificity, LC/MS/ MS detected the presence of 6-monoacetylmorphine, morphine or dihydrocodeine in all but two of 49 samples positive for opiates. Of 55 samples positive for benzodiazepines by ELISA, all but two were confirmed by LC/MS/MS. Overall, LC/MS/MS compared favourably with ELISA for detection of specific drugs or their metabolites in the case of morphine, methadone and the cocaine metabolite benzoylecgonine. Many of the discrepant results between the two assays were a result of samples with drug concentrations near to the cut-off concentrations and the imprecision of these assays at very low concentrations. Conclusion: LC/MS/MS offers a more flexible, specific and sensitive alternative to the screening of oral fluid samples for drugs of abuse than ELISA. A wide range of drugs and metabolites can be detected from a single sample injection. [ABSTRACT FROM AUTHOR]