What medications can cause a false positive result for Phencyclidine (PCP) on a drug screen?

Medications That Can Cause False Positive Results for PCP on Drug Screens

Several medications can cause false positive results for phencyclidine (PCP) on urine drug screens, including venlafaxine, desvenlafaxine, lamotrigine, dextromethorphan, diphenhydramine, tramadol, and certain other medications due to cross-reactivity with immunoassay screening tests.

Common Medications Causing False Positive PCP Results

Antidepressants

• Venlafaxine (Effexor) - Explicitly mentioned in FDA drug labeling as causing false positive PCP results 1
• Desvenlafaxine (Pristiq) - As the active metabolite of venlafaxine, also causes false positive PCP results 2

Anticonvulsants

• Lamotrigine (Lamictal) - Documented to cause false positive PCP results on Bio-Rad TOX/See urine screens 3

Over-the-Counter Medications

• Dextromethorphan (cough suppressant) - Significantly associated with false positive PCP results 4, 5
• Diphenhydramine (Benadryl) - Associated with false positive PCP results, though statistical significance varies between studies 4, 5

Pain Medications

• Tramadol - Significantly associated with false positive PCP results 4, 5

Other Medications

• Alprazolam, Clonazepam, Carvedilol - Associated with false positives, but may be confounded by polysubstance use 4

Understanding False Positive Results

Mechanism

• False positives occur due to lack of specificity in immunoassay screening tests
• Chemical structures similar to PCP can cross-react with antibodies used in these tests
• Standard immunoassays lack specificity to distinguish between structurally similar compounds 6

Duration of False Positive Results

• False positive test results may persist for several days following discontinuation of medications like venlafaxine 1

Clinical Implications

Confirmation Testing

• Confirmatory tests such as gas chromatography/mass spectrometry (GC-MS) will distinguish actual PCP from medications causing false positives 1
• When unexpected positive results occur, confirmation testing should be performed to avoid misdiagnosis 6

Patient Management

• False positive results can lead to inappropriate patient management if not properly identified
• Understanding which medications cause false positives is crucial for correct interpretation of drug screen results
• Patients taking medications known to cause false positives should inform healthcare providers before drug testing

Avoiding Misinterpretation

Best Practices

1. Obtain complete medication history, including prescription and OTC medications
2. Consider the possibility of false positives with certain medications
3. Use confirmatory testing when results will change treatment approach
4. Establish relationships with laboratory toxicologists who can assist with interpretation

Common Pitfalls

• Assuming a positive screen indicates illicit drug use without confirmation
• Failing to consider medication-induced false positives
• Taking punitive action based solely on screening results without confirmation
• Not recognizing that rapid screening tests have lower specificity than confirmatory methods

Remember that a positive drug screen should prompt a conversation with the patient rather than immediate punitive action, especially when the patient is taking medications known to cause false positive results.