The approach to management of a patient who incurs a positive urine drug test (UDT) screen for an illicit substance is complex. It is even more complex, however, if false positives could be the explanation.

Fischer et al report on their experience with 40 psychiatric patients found to be phencyclidine (PCP, also called “angel dust”) positive on UDT. Out of this population, only one patient confirmed taking PCP. The others were receiving psychiatric medications known to potentially produce a false-positive result for PCP. The authors report that the list of medications potentially causing a false-positive PCP UDT — most of which are used for psychiatric disorders — is substantial, and includes lamotrigine, tramadol, ibuprofen, imipramine, diphenhydramine, venlafaxine, and others.

This particular report, however, draws attention to another psychiatric medication, chlorprothixene, which they found to be the most common cause of a false-positive UDT for PCP, being associated with 16 of the 40 cases (venlafaxine was associated with 14 cases). Chlorprothixene has not previously been reported as a cause of false-positive PCP UDT.

These results must be considered preliminary because the investigators did not confirm the absence/presence of actual PCP by a highly sensitive method such as liquid chromatography. Nonetheless, these findings support consideration of chlorprothixene and other commonly used psychiatric drugs as the cause of false-positive PCP UDT results. Positive PCP results on UDT in a patient who denies using PCP may require confirmation with more sensitive assays than are typically used in routine UDT.