How can I determine the source of a positive phencyclidine (PCP) result in a urine drug screen?

Determining the Source of a Positive PCP Result on Urine Drug Screen

Always obtain confirmatory testing with gas chromatography-mass spectrometry (GC-MS) for any positive PCP immunoassay screen, as false-positives are common and confirmatory tests are highly specific for distinguishing true PCP use from cross-reactivity with medications. 1

Understanding PCP Testing Methodology

• Screening immunoassays (point-of-care or laboratory-based) are sensitive but susceptible to cross-reactions, resulting in false-positive results that limit their specificity. 1
• Confirmatory testing using GC-MS can positively identify PCP and is highly unlikely to yield false-positive results. 1, 2
• Results are classified as either "confirmed" (true positive) or "failed to confirm" (false positive) after GC-MS analysis. 3

Common Causes of False-Positive PCP Screens

Medications with Strong Evidence

Tramadol is the most well-documented cause of false-positive PCP screens, with both tramadol and its active metabolite N-desmethyltramadol confirmed to trigger false-positives through in vitro testing. 3, 4

Dextromethorphan is significantly associated with false-positive PCP results on immunoassay screens. 3

Venlafaxine and desvenlafaxine (Pristiq, Effexor) have been confirmed to cause false-positive PCP results on urine drug screens. 5, 6

Lamotrigine (Lamictal) has been reported to cause false-positive PCP results specifically on Bio-Rad TOX/See urine toxicology screens. 7

Medications with Possible Association

• Diphenhydramine may cause false-positives, though the association is not statistically significant in all studies. 3
• Alprazolam, clonazepam, and carvedilol showed statistical association with false-positive PCP screens, but these were also associated with polysubstance abuse, making causation unclear. 3

Clinical Algorithm for Interpretation

Step 1: Obtain Complete Medication History

• Document all prescription medications, over-the-counter drugs, and supplements the patient is currently taking. 1
• Specifically inquire about tramadol, dextromethorphan (cough medications), venlafaxine/desvenlafaxine, and lamotrigine use. 3, 4, 5, 6

Step 2: Assess Specimen Validity

• Check specimen temperature within 4 minutes of collection (should be 90°F-100°F or 32°-38°C). 1
• Order creatinine concentration with every sample; levels between 2-20 mg/mL indicate dilute specimens that may miss substances present in lower concentrations. 1
• Document specimen appearance and color to identify potential adulteration or substitution. 1
• Specimens that are substituted or adulterated should always be considered "positive" and may represent serious substance use disorder requiring specialist referral. 1

Step 3: Order Confirmatory Testing

• All positive PCP immunoassay screens require GC-MS confirmation to distinguish true positives from false-positives due to medication cross-reactivity. 1, 3, 2
• If the patient's clinical presentation does not match PCP intoxication and they are taking known interfering medications, strongly suspect a false-positive. 7, 6

Step 4: Interpret Results in Clinical Context

• If GC-MS confirms PCP: True positive indicating actual PCP use. 3
• If GC-MS fails to confirm: False-positive likely due to medication cross-reactivity. 3, 4
• PCP has unpredictable excretion patterns and can result in negative urines followed by positive urines without further ingestion due to redistribution from tissue stores. 8

Critical Pitfalls to Avoid

• Never make clinical decisions based solely on immunoassay screening results without confirmatory testing, as cross-reactivity is common. 1
• Do not dismiss the possibility of false-positives even when the patient denies medication use—obtain comprehensive medication history including recent medication changes. 1
• Be aware that different immunoassay platforms may have different cross-reactivity profiles (e.g., lamotrigine specifically affects Bio-Rad TOX/See screens). 7
• Remember that timing matters: PCP may not be detected in blood or urine if present only in nanogram/mL amounts, requiring sensitive methods for detection. 8

When Confirmatory Testing is Not Immediately Available

• Seek assistance from the testing laboratory when results do not correlate with clinical findings. 1
• Consider repeat testing if substance use disorder is highly suspected clinically despite negative or questionable results. 1
• Document all interfering medications in the medical record to provide context for interpretation. 1