Your patient presents unresponsive with an empty bottle of alprazolam (Xanax). You order a urine and blood toxicology screen. The blood comes back negative for benzodiazepines but the urine test is positive. How do you interpret this result?
The benzodiazepine toxicology screen typically looks for oxazepam. If it is present in sufficient quantity, the test will be positive.
Three benzodiazepines are detected by this test: oxazepam (Serax), diazepam (Valium), and chlordiazepoxide (Librium); [diazepam and chlordiazepoxide are metabolized to oxazepam].
Other benzodiazepines such as clonazepam, lorazepam, and alprazolam will generally test negative unless there is cross-reactivity or large quantity.
The urine and blood immunoassays are exactly the same. For this patient, there was probably a low overall quantity of alprazolam in the blood but a concentrated amount in the urine. Therefore, the positive urine and negative blood.
