Using 2 testing methods is a general requirement in forensic drug testing; the methods must be based on different scientific principles to aid the laboratory report a positive result. The initial test is an immunoassay for regulated drug testing; the confirmation test is GC-MS.
Weighing cost against analytical quality factors, such as detection of limits and selectivity, is the first challenge for a laboratory in selecting an initial test. The ability to test multiple specimens rapidly is embedded in cost. Most common in the drug-testing industry are homogeneous immunoassays since they are comparatively cheap and can adapt to automated high-volume analyzers that can test over 700 specimens per hour. Ensuring accurate specimen identification, these methods also make testing more affordable for the customers. Reading barcodes on aliquot tubes, many analyzers associate the test results directly with the specimen number in the LIMS data file, thus making tube-switching irrelevant.
Having the drug of interest at the cutoff concentration, in a matrix that responds in the same manner as specimens, is important to be constituted in the initial test calibrators. Calibrator’s concentration is typically required to be at ±10%, as expected by the GC-MS. Certified by the manufacturer, many calibrators accompany the immunoassay kit; however, verifying this certification must be done by each laboratory for its own validation.
Extracting drug from urine and reconstituting it in a solvent for injection into the instrument constitutes the GC-MS analyses. Analogous to the condition of the specimen, the calibrator and QC samples must be tested contemporaneously. Once high-quality procedures are selected and assays are validated, the next QA step is to ensure that analysts conduct the test with premium practices. Though good training and supervision assures this, there are specific checks on the testing process that have become accepted as standard practices. To ensure that the MS is working properly, instruments are auto tuned daily and the printouts are kept as a record for verification. Recording auto tunes, that fail when other auto tunes on the same day were successful, is also included which allows identifying trends in deteriorating performance of instruments by the managers and QA personnel. Deuterated internal standard are needed. A criterion to evaluate peak shape and resolution should also be drafted by the laboratory. The solvents in wash vials must be changed frequently for instruments in auto samplers. The time is usually <1 day, however the frequency is not specified. Nonetheless laboratories should avoid the practice of adding solvent several times, as the solvent level in wash vials decreases, instead of dumping and refilling.