What can cause a false positive for benzoylecgonine (BZE), a metabolite of cocaine?

False Positives for Benzoylecgonine (Cocaine Metabolite)

Several substances and factors can cause false positive results for benzoylecgonine in drug tests, including certain medications, foods, and testing issues. 1

Common Causes of False Positives

Medications and Substances

• Cross-reactive medications:
  • Fluoroquinolone antibiotics (reported to cross-react with immunoassay opiate screens, which may affect multi-panel tests) 2
  • Pseudoephedrine
  • Dextromethorphan
  • Some antihistamines
  • Some antidepressants 1

Testing Issues

• Immunoassay limitations:
  • Standard immunoassay tests are susceptible to cross-reactions causing false positives 1
  • 81% of cocaine-only positive specimens tested by both LC-MS-MS and immunoassay were negative by immunoassay, demonstrating limitations of screening with benzoylecgonine-specific immunoassays 3

Foods

• Poppy seeds can cause false-positive results for cocaine in drug tests 1

Sample Adulteration

• Adulterated or substituted samples can affect test results
• Diluted urine samples may affect detection thresholds 1

Confirming Test Results

Gold Standard Confirmation Methods

• Gas chromatography/mass spectrometry (GC-MS) or liquid chromatography/mass spectrometry (LC-MS/MS) are the gold standard for confirming positive immunoassay results 1
• These laboratory-based methods can positively identify specific substances and concentrations but are higher in cost 1

Interpreting Results

• When interpreting drug test results, consider:
  1. Timing of potential drug use relative to testing
  2. Current medications the patient is taking
  3. Cutoff concentration thresholds that may miss small amounts 2

Clinical Considerations

Detection Windows

• Benzoylecgonine has a urinary half-life of 6-8 hours
• Can typically be detected in urine for 24-48 hours after cocaine use
• In heavy users (up to 10g/day), benzoylecgonine has been detected up to 22 days after last ingestion 2

Avoiding False Positives

• Always document any medications the patient is currently taking to help interpret results 1
• When unexpected positive results occur, discuss with the patient in a non-judgmental manner rather than taking immediate punitive action 1
• Consider confirmatory testing using GC-MS or LC-MS/MS when immunoassay results are positive but clinical history doesn't support cocaine use 1

Special Considerations

• Unlike some other substances, lidocaine and its metabolite norlidocaine (MEGX) do not appear to cause false positive results for cocaine/benzoylecgonine on standard urine immunoassays 4
• Testing for parent cocaine in addition to benzoylecgonine may be important, as 3% of positive specimens in one study showed cocaine without benzoylecgonine, which could lead to false negatives if only testing for the metabolite 3

Pitfalls to Avoid

• Don't rely solely on immunoassay screening without confirmation for important clinical or legal decisions
• Don't dismiss the possibility of false positives when clinical presentation doesn't match test results
• Don't forget that certain medications or foods can interfere with test results
• Don't overlook the possibility that a negative test doesn't necessarily mean absence of use if testing occurred outside the detection window 2