Managing False Positive Results for Amphetamines
False positive amphetamine results on urine drug screens should be confirmed using gas chromatography/mass spectrometry (GC-MS) or liquid chromatography/mass spectrometry (LC-MS/MS), which are the gold standard confirmatory tests. 1
Common Causes of False Positive Amphetamine Results
Immunoassay-based drug screens, while fast and relatively inexpensive, are susceptible to cross-reactions that can cause false-positive results for amphetamines. Several medications have been documented to cause false positive amphetamine results:
- Bupropion: This is the most frequent cause of false positive amphetamine screens, with studies showing it accounts for 41% of false positive results 2
- Pseudoephedrine and other decongestants 1, 3
- Antipsychotics: Including aripiprazole 4
- Antidepressants: Various classes can cross-react 1
- Other medications: Imatinib 5, dextromethorphan, some antihistamines 1
Management Algorithm for False Positive Amphetamine Results
Step 1: Documentation and Patient Discussion
- Document all medications the patient is currently taking, including prescription, over-the-counter, and supplements 1
- Discuss unexpected positive results with the patient in a non-judgmental manner
- Never take punitive action based solely on an immunoassay screening result 1
Step 2: Confirmatory Testing
- Order confirmatory testing with GC-MS or LC-MS/MS for definitive results 1, 6
- The FDA label for bupropion specifically states: "Confirmatory tests, such as gas chromatography/mass spectrometry, will distinguish bupropion from amphetamines" 6
Step 3: Interpretation of Results
- If confirmatory testing is negative, document the false positive result and its likely cause
- If the patient is on bupropion, note that false-positive test results may persist even after discontinuation of therapy 6
- Be aware that false positives make up 3.9-9.9% of positive amphetamine screens in clinical laboratories 7
Clinical Implications and Pitfalls
- Avoid misdiagnosis: False positive results can lead to unnecessary treatment and monitoring 4
- Prevent patient stigmatization: Incorrect labeling of patients as substance users can damage the provider-patient relationship
- Medication reconciliation: Always review the patient's complete medication list before interpreting drug screen results
- Documentation: Clearly document in the medical record when a false positive is suspected or confirmed
- Patient education: Inform patients taking medications known to cause false positives about this possibility
Special Considerations
- In patients on chronic medication monitoring, consider using confirmatory testing directly rather than immunoassay screening if false positives are a concern
- For patients with substance use disorders in recovery programs, false positives can have serious consequences for treatment and legal status, making confirmatory testing essential
- Healthcare providers should never dismiss patients from care based solely on urine drug test results, as this could constitute patient abandonment 1
By following this structured approach, clinicians can effectively manage false positive amphetamine results, ensure accurate interpretation, and maintain appropriate patient care.