Can omeprazole cause false positive results for fentanyl or hydroxyzine?

Omeprazole Does Not Cause False Positive Results for Fentanyl or Hydroxyzine

Omeprazole has not been documented to cause false positive results for either fentanyl or hydroxyzine in urine drug screens. While immunoassay-based drug tests are known to have cross-reactivity issues leading to false positives, there is no evidence in the current literature linking omeprazole to false positive results for these specific substances.

Understanding Drug Test Cross-Reactivity

Immunoassay-based drug tests, which are commonly used for initial screening, have several limitations:

• They are susceptible to cross-reactions, resulting in false-positive results that limit their specificity 1
• Qualitative tests (immunoassays) provide positive/negative results but are prone to false positives 2
• Confirmatory testing using gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS) is recommended to verify positive screening results 2

Known Causes of False Positive Fentanyl Results

Several medications have been documented to cause false positive results for fentanyl:

• Risperidone (Risperdal), particularly the long-acting injectable form (Risperdal Consta), has been reported to cause false positive fentanyl results 3, 4
• Other substances that may cause false positive fentanyl results include:
  • Diphenhydramine
  • Lidocaine
  • MDMA
  • Methamphetamine 5

Omeprazole's Drug Interaction Profile

Omeprazole is known to interact with the cytochrome P450 system in the liver, but these interactions are primarily pharmacokinetic rather than analytical:

• It can inhibit the metabolism of drugs metabolized by the cytochrome P450 enzyme subfamily IIC (diazepam, phenytoin) 6, 7
• It does not affect the metabolism of drugs metabolized by subfamilies IA (caffeine, theophylline), IID (metoprolol, propranolol), and IIIA (cyclosporin, lidocaine, quinidine) 7
• These interactions affect drug metabolism in the body, not the analytical performance of drug tests

Interpreting Drug Test Results

When interpreting drug test results:

1. Consider the limitations of immunoassay screening tests
2. Document all medications the patient is currently taking to help interpret results 2
3. For unexpected positive results:
   • Initiate a conversation with the patient rather than taking immediate punitive action 2
   • Consider confirmatory testing using GC-MS or LC-MS, which can differentiate between various substances and provide more accurate results 2

Key Points for Clinical Practice

• Standard drug screens have detection windows that may miss substances used outside that timeframe
• Standard panels don't test for all possible substances
• Some drugs require specific testing not included in standard panels 2
• The positive predictive value of fentanyl immunoassays has been reported as 85.7% in one study 2
• Confirmatory testing should be performed for all samples that screen positive for fentanyl 4

In summary, while drug test cross-reactivity is a legitimate concern when interpreting results, there is no evidence that omeprazole causes false positive results for either fentanyl or hydroxyzine. If a patient taking omeprazole tests positive for either substance, other causes should be investigated, and confirmatory testing should be considered.