What substances can cause a false positive result in immunoassay testing for opiates?

False Positives in Immunoassay Testing for Opiates

Several medications and substances can cause false positive results in immunoassay testing for opiates, including quinolone antibiotics (particularly levofloxacin, ofloxacin, and pefloxacin), rifampin, and poppy seeds. 1, 2, 3

Common Causes of False Positive Opiate Immunoassay Results

Medications

• Quinolone antibiotics:

  • Most likely to cause false positives: levofloxacin, ofloxacin, and pefloxacin 3
  • Other quinolones with documented cross-reactivity: enoxacin, moxifloxacin, gatifloxacin, trovafloxacin, sparfloxacin, lomefloxacin, ciprofloxacin, clinafloxacin, and norfloxacin 3

• Rifampin:

  • Can cross-react with KIMS (Kinetic Interaction of Microparticles in Solution) method immunoassays 2
  • Specifically mentioned in FDA labeling as causing false positives in tests like Abuscreen OnLine opiates assay 2

• Other medications:

  • Pseudoephedrine
  • Dextromethorphan
  • Some antihistamines
  • Some antidepressants 1

Food Products

• Poppy seeds: Can trigger false positive results 1

Detection Limitations and Testing Considerations

Immunoassay Limitations

• Standard point-of-care tests are qualitative immunoassays that detect drug classes rather than specific medications 1
• These tests have limitations in both sensitivity and specificity 1
• Different immunoassay systems show varying cross-reactivity patterns with the same substances 3, 4

False Positive Rates

• False positive rates for opiate immunoassays can be as high as ~34% 4
• This high rate underscores the importance of confirmatory testing

Confirmatory Testing Recommendations

When to Confirm Results

• The CDC recommends confirmatory testing when:
  • There's a need to detect specific opioids not identifiable on standard immunoassays
  • Unexpected results are present
  • Results are inconsistent with clinical expectations 1

Confirmatory Methods

• Gold standard methods:
  • Gas chromatography/mass spectrometry (GC-MS)
  • Liquid chromatography/mass spectrometry (LC-MS/MS) 1
• These methods can definitively identify specific substances and their metabolites 1

Clinical Approach to Unexpected Positive Results

1. Discuss with patient about potential causes of false positives before taking action 1
2. Review medication history for agents known to cause false positives
3. Consider dietary factors like poppy seed consumption
4. Order confirmatory testing when clinical picture doesn't match immunoassay results
5. Never dismiss patients from care based solely on urine drug test results 1

Important Caveats

• Different immunoassay brands have different cross-reactivity profiles 3, 4
• The absence of a substance on a drug test doesn't definitively prove non-use; it may reflect test limitations or timing issues 1
• Detection windows vary by substance, with most opioids detectable for only 1-3 days after use 1
• Confirmatory testing should be performed before making significant clinical decisions based on unexpected positive results 1, 2