Can topical lidocaine use cause a false positive urine test for cocaine?

Topical Lidocaine Does Not Cause False Positive Urine Tests for Cocaine

Topical lidocaine does not cause false positive results on urine drug screens for cocaine. This is confirmed by direct research evidence showing no instances of false positives among patients receiving lidocaine 1.

Understanding Drug Testing and False Positives

Types of Drug Testing

• Urine drug testing is the most common method used in clinical settings due to being well-standardized, less invasive than blood testing, and providing a longer detection window 2
• Two main types of drug tests are used:
  • Qualitative tests (screening): Point-of-care immunoassays that give rapid results but are susceptible to cross-reactions
  • Confirmatory tests: Laboratory-based tests using gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS) that provide definitive results 2, 3

Evidence on Lidocaine and Cocaine Testing

A comprehensive study specifically investigating this question enrolled 168 subjects receiving lidocaine as part of their medical care 1:

• 121 samples were positive for lidocaine
• 114 of these were also positive for norlidocaine (MEGX), lidocaine's primary metabolite
• None of the samples tested positive for cocaine/benzoylecgonine on standard urine immunoassays
• The 95% confidence interval was 0-3.9% for false positives among those with confirmed lidocaine metabolites

Known Causes of False Positives for Cocaine

While lidocaine does not cause false positives for cocaine, other substances can trigger false-positive results on drug screening tests:

• Certain medications including some antidepressants, antipsychotics, and antibiotics can cause false positives for various substances 4
• Substances that have been documented to cause false positives include:
  • Diphenhydramine at high concentrations (≥1 mg/mL) 5
  • Various antidepressants and antipsychotics 4

Clinical Implications

Interpreting Unexpected Positive Results

• Always document medications the patient is currently taking to help interpret results 3
• Unexpected positive results should prompt a conversation with the patient rather than immediate punitive action 3
• Confirmatory testing using GC-MS or LC-MS is recommended to verify positive screening results 3

Avoiding Common Pitfalls

• Do not dismiss patients from care based solely on urine drug test results 3
• Establish a working relationship with laboratory toxicologists to assist with result interpretation 3
• Be aware that while lidocaine does not cause false positives for cocaine, other medications may cause false positives for various substances

Bottom Line

Despite anecdotal claims, scientific evidence demonstrates that topical lidocaine use does not cause false positive results on cocaine urine drug screens. If a patient tests positive for cocaine while using lidocaine, clinicians should consider other explanations, including actual cocaine use or interference from other medications known to cause false positives.