From the Guidelines
Substances that cause false positives for phencyclidine (PCP) include dextromethorphan, an over-the-counter cough suppressant, and venlafaxine, an antidepressant, as they are known to cause false positives for PCP in certain urine drug screens 1.
- Additionally, diphenhydramine, an antihistamine, and tramadol, a pain medication, may also lead to false positive results for PCP.
- It is essential to consider these potential cross-reactants when interpreting urine drug screen results to avoid misdiagnosis or misinterpretation, as preliminary screening tests may yield more false-positive results compared to confirmatory tests 1.
- Understanding the type of drug test used and being aware of the limitations of the selected matrix, substances tested, and potential cross-reactivity is crucial for accurate interpretation of drug test results 1.
- A patient's complete medical history, including prescribed medications, should be considered when interpreting drug test results to distinguish between appropriate use and misuse of prescribed medications 1.
From the FDA Drug Label
False-positive urine immunoassay screening tests for phencyclidine (PCP) and amphetamine have been reported in patients taking venlafaxine This is due to lack of specificity of the screening tests. False positive test results may be expected for several days following discontinuation of venlafaxine therapy.
- Venlafaxine can cause false positives for phencyclidine (PCP) and amphetamine due to the lack of specificity of the screening tests 2.
- Confirmatory tests, such as gas chromatography/mass spectrometry, can distinguish venlafaxine from PCP and amphetamine.
From the Research
Substances Causing False Positives for Phencyclidine (PCP)
- Tramadol, dextromethorphan, and diphenhydramine have been associated with false-positive urine screens for PCP 3
- Alprazolam, clonazepam, and carvedilol may also cause false-positive PCP screens, but this is often associated with polysubstance abuse 3
- Dextromethorphan, in particular, has been shown to produce PCP-like psychological symptoms, including delusions, hallucinations, and paranoia, when consumed at high doses 4
- Other substances, such as levorphanol, may be difficult to distinguish from dextromethorphan in urine tests, potentially leading to false positives for PCP 5
Mechanisms and Associations
- Dextromethorphan's antagonism at N-methyl-d-aspartate receptors (NMDAr) is thought to underlie its efficacy in treating acute cough, but supratherapeutic doses can mimic the activity of dissociative hallucinogens, such as phencyclidine and ketamine 6
- The use of certain medications, such as tramadol and dextromethorphan, can increase the likelihood of false-positive PCP screens 3
- Polysubstance abuse can also contribute to false-positive PCP screens, particularly with substances like alprazolam, clonazepam, and carvedilol 3