Immunoassay and confirmation testing using Gas chromatography and some drugs that result in false positives using immunoassay testing only

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Immunoassay and confirmation testing using Gas chromatography and some drugs that result in false positive’s using immunoassay testing only. Also a short list of drugs known to cause false positives when tested using immunoassay.

Submitted: May 05, 2015

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Submitted: May 05, 2015





Immunoassay and confirmation testing using Gas chromatography and some drugs that result in false positive’s using immunoassay testing only.

Two different assays are commonly available for urine drug testing. The immunoassay is quick, highly sensitive, and relatively inexpensive but may lack specificity. It tests for classes of drugs (such as opiates) without distinguishing among individual drugs within that class. Gas chromatography in combination with mass spectrometry (GC-MS) is a more expensive and time-consuming test, but is the gold standard for confirming a positive result on immunoassay. By definition, all positive results on GC-MS are true positives.

Reports of false-positive urine drug screening for substances of abuse are infrequent and limited to case reports and a few controlled-exposure studies. TheTable lists some of the substances reported to cause false-positive results.

Positive confirmation tests may occur in urine specimens from patients who legally or unknowingly ingest products that contain drugs of abuse. In these instances, the finding is a true positive but may not reflect drug abuse by the client. Many products available without prescription outside of the US contain opiates (e.g., Donnagel PG from Canada).1 Several controlled-exposure studies have shown that as little as 1 poppy seed muffin (about 15 g of seed) can produce detectable amounts of morphine and codeine by immunoassay as well as GC-MS.1,2 In 1998, the federal government increased the threshold defining a positive screen for urine morphine and codeine from 300 to 2000 ng/mL to reduce spurious reports of opiate-positive tests from poppy seed consumption.1,2

Substances that do not produce positive urine drug screens include passively inhaled crack cocaine or marijuana (unless “extreme”), and ingested products containing hemp or other common herbal preparations.1,2,10 In one study, 6 volunteers in an 8×8×7-ft enclosed room were exposed to 200 mg freebase cocaine vapor; none of their urine samples exceeded the federal GC-MS threshold. In a similar study of 3 non-smokers exposed to 8 marijuana smokers (smoking 32 joints) in a 10×10×8-ft enclosed room, no samples from the nonsmokers exceeded the federal GC-MS threshold.2 In an exposure study of 90 volunteers who ingested 8 different herbal preparations, there were no positive urine drug screens.1


Substances reported to cause false-positive urine drug screen results





Amphetamine and methamphetamine


Single case report1,2

L-stereoisomer only detected (D-stereoisomer present in illicit drugs)

Amphetamine and methamphetamine

Vicks Inhaler

Several case reports, controlled-exposure studies1-3

L-stereoisomer only detected; most positives noted with twice recommended dosage


NSAIDs (ibuprofen, naproxen)

Controlled-exposure study of 60 subjects (510 specimens)4

0.4% false-positive rate



Controlled-exposure study of 12 patients (36 specimens)5

100% false-positive rate, some cases lack controls


NSAIDs (ibuprofen, naproxen)

Controlled-exposure study of 60 subjects (510 specimens)4

0.4% false-positive rate



Controlled-exposure studies (8 subjects) and case series (9 subjects)6

Most levels detected were below new 1998 threshold (2000 ng/mL)



3 case reports7




1case report8

Confirmed by GC-MS (7200 mg intentionally ingested)



1case report9

(500 mg ingested)

*Ofloxacin and levofloxacin most likely to cause false positive.


Recommendations from others

The US Department of Health and Human Services requires confirmation of positive immunoassay results by GC-MS for drug testing in the workplace.1 The College of American Pathologists, the principal organization of board-certified pathologists, states: “Confirmation testing, a standard of practice in forensic toxicology should be performed in clinical toxicology whenever possible.


  1. Medical Review Officer Manual for Federal Agency Workplace Drug Testing Programs. US Department of Health and Human Services, Substance Abuse and Mental Health Services Administration, Division of Workplace Programs. Available at: Accessed on September 6, 2006.
  2. elSohly MA, Jones AB. Drug testing in the workplace: could a positive test for one of the mandated drugs be for reasons other than illicit use of the drug? J Anal Toxically 1995;19:450–458.
  3. Poklis A, Moore KA. Response of EMIT amphetamine immunoassays to urinary desoxyephedrine following Vicks inhaler use. There Drug Monit 1995;17:89–94.
  4. Rollins DE, Jennison TA, Jones G. Investigation of interference by nonsteroidal anti-inflammatory drugs in urine tests for abused drugs. Clin Chem 1990;36:602–606.
  5. Fraser AD, Howell P. Oxaprozin cross-reactivity in three commercial immunoassays for benzodiazepines in urine. J Anal Toxicol 1998;22:50–54.
  6. Zacher JL, Givone DM. False-positive urine opiate screening associated with fluoroquinolone use. Ann Pharmacother 2004;38:1525–1528.
  7. Daher R, Haidar JH, Al-Amin H. Rifampin interference with opiate immunoassays. Clin Chem 2002;48:203–204.
  8. Bond GR, Steele PE, Uges DR. Massive venlafaxine overdose resulted in a false positive Abbott AxSYM urine immunoassay for phencyclidine. J Toxicol Clin Toxicol 2003;41:999–1002.
  9. Budai B, Iskandar H. Dextromethorphan can produce false positive phencyclidine testing with HPLC. Am J Emerg Med 2002;20:61–62.
  10. Markowitz JS, Donovan JL, DeVane CL, Chavin KD. Common herbal supplements did not produce false-positive results on urine drug screens analyzed by enzyme immunoassay. J Anal Toxicol 2004;28:272–273.
  11. Caplan YH, Kwong TC. Evaluation of Toxicology Test Results. Available at: Accessed on September 6, 2006.



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