What medications can cause a false positive Urine Drug Screen (UDS) for Phencyclidine (PCP)?

Medications That Can Cause a False Positive Urine Drug Screen for PCP

Venlafaxine (Effexor) and its active metabolite desvenlafaxine (Pristiq) are the most commonly documented medications that cause false positive results for phencyclidine (PCP) on urine drug screens. 1, 2, 3

Common Medications Associated with False Positive PCP Results

Strongly Associated:

• Antidepressants:
  • Venlafaxine (Effexor) 1, 4
  • Desvenlafaxine (Pristiq) 2

Possibly Associated:

• Other Medications:
  • Dextromethorphan (found in many OTC cough medicines) 5, 6
  • Tramadol 5
  • Diphenhydramine (Benadryl) 5, 6
  • Alprazolam (Xanax) - though may be associated with polysubstance abuse 5
  • Clonazepam (Klonopin) - though may be associated with polysubstance abuse 5
  • Carvedilol - though may be associated with polysubstance abuse 5

Mechanism of False Positive Results

The false positive results occur due to the lack of specificity in immunoassay screening tests. The molecular structure of certain medications or their metabolites can cross-react with the antibodies used in the PCP immunoassay, leading to false positive results 1.

Clinical Implications

1. Confirmatory Testing:

   • Always confirm positive PCP screening results with more specific methods such as gas chromatography/mass spectrometry (GC/MS) 1, 4
   • False positive test results may persist for several days after discontinuation of the causative medication 1

2. Documentation:

   • When ordering urine drug screens, document all medications the patient is taking, including prescription and over-the-counter medications
   • Inform laboratory personnel about medications that might cause false positive results

3. Patient Education:

   • Inform patients taking these medications about the possibility of false positive drug screens
   • This is particularly important for patients subject to workplace drug testing or legal monitoring

Concentration Effects

The likelihood of false positive results may depend on medication concentration:

• In the case of venlafaxine, very high concentrations (such as in overdose situations) are more likely to cause false positives 4
• However, false positives have also been documented with therapeutic doses, particularly with long-term use 3

Special Considerations

• Testing Methods: Different immunoassay products have varying cross-reactivity profiles. The Abbott AxSYM, Siemens Syva EMIT II, and Medtox immunoassays have all been documented to produce false positive PCP results with certain medications 2, 5, 4

• Polysubstance Use: When evaluating positive PCP screens in patients taking alprazolam, clonazepam, or carvedilol, consider the possibility of polysubstance abuse as a confounding factor 5

• Pediatric/Adolescent Patients: False positive PCP results have been documented in adolescents taking venlafaxine at therapeutic doses 3

When a positive PCP result is unexpected, always consider medication-induced false positives before making clinical or legal decisions based on the screening result.