What Can Cause A Positive Drug Test For PCP?

In our latest question and answer, the pharmacist discusses what drugs could potentially cause a false positive drug test for PCP.

Question

I take Ibuprofen 5-6 times a day and 300 mg tramadol 4-5 times per day. I just received a urine drug test back that was positive for PCP. I’m a 53 yr old grandma who doesn’t smoke or drink even a beer. I don’t even know what PCP is. What should I do?

Asked by Sheila On Jan 05, 2018

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Jan 05, 2018

PCP (Phencyclidine) is an illicit substance that is commonly abused and is known for its dissociative effects. It is commonly tested for on drug screenings. In fact, PCP is one of the 8 most commonly tested for substances and is included on most standard urinalysis screenings. Other commonly tested substances include:

  • Amphetamines
  • Cocaine
  • Benzodiazepines
  • Barbiturates
  • Marijuana 
  • Opiates
  • Opioids


What Causes A False Positive PCP Test?

What has apparently happened in your situation is known as a "false positive", a mistaken positive reading on the drug test which is not accurate. Unfortauntely, false positives are relatively common on standard urinalysis drug tests.


The issue with common drugs tests is that they aren't all that accurate. Most drug screening facilities use simple "immunoassay" based tests that work by utilizing antibodies to react to specific drug compounds. When a specified drug is present in a test urine sample, an antibody binds to it, producing a reaction. When a reaction occurs, it is recorded as a 'positive' result.


These "immunoassay" based tests can have significant problems with "cross-reactivity". In other words, drugs that are NOT being tested for can occasionally react with the wrong antibody (i.e. 'cross react'), resulting in a 'false positive'. 


There are many non-illicit, legal medications which are well known to cause positive tests. In your situation, you are unfortunately taking two medications which are specifically known to cause false positive PCP tests. The following medications are known and documented as being able to produce false positive PCP tests:

  • Dextromethorphan
  • Diphenhydramine
  • Ibuprofen
  • Imipramine
  • Ketamine
  • Meperidine
  • Thioridazine
  • Tramadol
  • Venlafaxine


If you are sure that your test result is incorrect and a false positive has occurred, you could ask to retake the test. In addition, you can request another type of drug test that does not have problems with cross-reactivity. Two of these tests are:

  • Gas chromatography-mass spectrometry (GC-MS)
  • High-performance liquid chromatography (HPLC)


Both of the above tests do not use antibodies and can accurately determine the exact molecules present in a sample. If you received a 'false positive', these tests will be able confirm that a particular drug compound is not present.

About the Pharmacist

Dr. Brian Staiger Pharm.D

Dr. Brian Staiger is a licensed pharmacist in New York State and the founder of PharmacistAnswers.com. He graduated from the University At Buffalo with a Doctor of Pharmacy degree in 2010. He has been featured in numerous publications including the Huffington Post as well as a variety of health and pharmacy-related blogs. Please feel free to reach out to him directly if you have any inquiries or want to connect! He's answered thousands of medication and pharmacy-related questions and he's ready to answer yours! Brian.Staiger@PharmacistAnswers.com Office: 716-389-3076

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