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Abstract
Case report: A 13‐yr‐old girl overdosed on 48 × 150 mg venlafaxine (Effexor XR®). She was taking venlafaxine regularly for depression. Her only other medications included topical Benzamycin and pyridoxine 50 mg daily for acne. The Abbott AxSYM® assay was positive only for phencyclidine, but GC/MS did not confirm the presence of phencyclidine. Toxilab® identified only one substance, confirmed by GC/MS as venlafaxine. A serum sample obtained 3 h after her ingestion revealed a venlafaxine concentration of 24460 ng/mL and an O‐desmethylvenlafaxine concentration of 3930 ng/mL, confirming the massive acute overdose (therapeutic range of venlafaxine and O‐desmethylvenlafaxine together is 250–750 ng/mL). Urine spiked with 4.2 mg/mL of venlafaxine and 0.7 mg/mL of O‐desmethylvenlafaxine was interpreted as positive with the Abbott AxSYM® fluorescent polarized immunoassay for phencyclidine (readout of 28 ng/mL). Conclusion: Venlafaxine may cause a false positive Abbott AxSYM® phencyclidine assay when present in very high concentrations. Physicians should be aware of this potential reaction when interpreting urine drug immunoassays.