The investigation determined that false negative vitros benz results (<85 ng/ml) were obtained from two different samples from the same patient based on confirmatory testing using gc/ms (1087 and 1960 mcg/l).The most likely cause is a known limitation of the vitros benz reagent related to low cross reactivity with lorazepam glucuronide.Literature sources indicate that unchanged lorazepam is excreted in negligible amounts in urine.The drug is conjugated almost immediately and builds up in plasma as the conjugated metabolite.The lorazepam conjugated metabolite (lorazepam glucuronide) is then slowly excreted over time into the urine.As stated in the vitros benz ifu, reference methods hydrolyze benzodiazepine glucuronides in the extraction process, increasing the recovery of benzodiazepines by the reference method as compared to the vitros benz reagent.It is likely the urine sample contained lorazepam glucuronide and the reference method used hydrolyzed the lorazepam glucuronide increasing the level of lorazepam in the sample that was not available for the vitros benz reagent to detect.At the 200 ng/ml cutoff, vitros benz reagent has < 2.0% cross reactivity with lorazepam glucuronide, and at the 300 ng/ml cutoff vitros benz reagent has <3.0% cross reactivity with lorazepam glucuronide.Lorazepam glucuronide >10,000 ng/ml would be required before the vitros benz reagent could detect it suggesting this compound is virtually undetectable using the vitros benz reagent.
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