GCMS Assay Validation Evaluation

SAMHSA-certified laboratories are required to validate assays prior to implementation and evaluate performance characteristics annually. For non-certified laboratories, inclusion of validation and periodic evaluation of confirmation assays as part of their quality assurance plan demonstrates a commitment to professionalism and good laboratory practice. Validation studies should include determination of upper limit of linearity, limit of detection, limit of quantitation, precision/accuracy around cutoff concentrations, and carryover and interference studies (43). Chromatographic evaluation of possible interferents should include potential for co-elution, presence of extraneous peaks, and similarity of mass ions and ion ratios to amphetamine and methamphetamine (42). SAMHSA-certified laboratories are required to perform interference studies for amphetamine confirmation assays by analyzing samples containing interferents (phentermine at 50,000 ng/mL and phenylpropanolamine, ephedrine, and pseudoephedrine at 1 mg/mL) in the presence of and without amphetamine and methamphetamine at 40% of the cutoff (43). Other compounds with similar structures that may be tested for interference include hydroxynorephedrine, norephedrine, norpseudoephedrine, phenylephrine, and propyhexedrine (41). If a certified laboratory performs isomer resolution assays, interference studies must include samples with d-methamphetamine, l-methamphetamine, d-amphetamine, and l-amphetamine at 100ng/mL each and samples without amphetamine or methamphetamine (43).

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