Massive venlafaxine overdose resulted in a false positive Abbott AxSYM urine immunoassay for phencyclidine.
METHODS
A 13-yr-old girl overdosed on 48 x 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 ofvenlafaxine 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).


CONCLUSIONS
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.
Bond
G Randall
GR
Drug and Poison Information Center, Department of Emergency Medicine, Children's Hospital Medical Center, Cincinnati, Ohio 45229, USA. randy.bond@cchmc.org
Steele
Paul E
PE
Uges
Donald R A
DR
D000293:Adolescent; D003511:Cyclohexanols / Q000097:blood / Q000506:poisoning* / Q000627:therapeutic use / Q000652:urine*; D003863:Depression / Q000188:drug therapy; D062787:Drug Overdose / Q000097:blood / Q000175:diagnosis / Q000652:urine; D005189:False Positive Reactions; D005260:Female; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D007118:Immunoassay; D010622:Phencyclidine / Q000652:urine*; D000069470:Venlafaxine Hydrochloride
D003511:Cyclohexanols; D000069470:Venlafaxine Hydrochloride; D010622:Phencyclidine