An immunoassay (IA) test is a type of urine drug test (UDT) that can screen for phencyclidine, which people also call PCP or angel dust. This testing method is quick, easy to use, and cost effective. However, false positives are possible.

Testing urine is a fast, convenient method to screen for phencyclidine and other substances. However, taking certain substances, including some prescription medications and supplements, may cause a false positive result.

A person needs to inform the test provider of any medication or supplements they are taking, including over-the-counter (OTC) and herbal products.

This article looks at how screening works, the screening results, how false positives may occur, and which medications may cause false positive phencyclidine results.

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In an IA UDT, a person typically urinates in a plastic container. A doctor or technician usually requires a minimum of 30 milliliters (mL) of urine for testing.

The technician or doctor who receives the urine sample may examine it for signs of dilution or attempts to make it impure.

Once they have ensured the sample is valid, technicians will use antibodies called assays to detect the presence of phencyclidine in the urine. The test can also detect the presence of other drugs, including amphetamines, cocaine, opiates, and cannabis.

An IA UDT can usually detect phencyclidine within 8 days of use.

The results of an IA UDT will either be negative or positive. The results are typically available either on the same day as the test or within a few days.

Negative result

If the results of the test are negative for phencyclidine, it means that the technicians or doctors:

  • did not find any phencyclidine in the urine sample
  • found a small amount of phencyclidine in the sample, but not enough to trigger a positive result

Positive result

If an IA UDT produces a positive result for phencyclidine, it means the technicians or doctors:

  • found a high enough amount of phencyclidine in the urine sample to cause a positive result, which could indicate the use or misuse of the drug
  • found the test indicated the use of phencyclidine when a person has not used the substance, which means it has produced a false positive result

Reports of false positive results for phencyclidine in UDT IA tests are common and have associations with several prescription and OTC medications. This may be due to the structural similarities between phencyclidine and parts of certain medications.

Certain compounds from legal medications in the urine sample may bind to the assay antibodies in the same way as phencyclidine compounds would, which can cause a false positive result.

IA UDTs are simplistic, straightforward tests that are highly sensitive but not highly specific. The test cannot distinguish between drugs within the same class and can be unreliable when certain substances are present.

If the results of an IA test are positive for phencyclidine, technicians or doctors may request a follow-up with another type of urine drug test, called a gas chromatography-mass spectrometry (GC-MS).

A GC-MS test is more labor-intensive and costly and requires advanced laboratory services. However, the test is more reliable, specific, and accurate.

A GC-MS test may help confirm a positive IA result for phencyclidine or confirm that a positive result from an IA UDT was a false positive.

Several medications may cause a false positive result for phencyclidine in a urine drug test. Some are common OTC drugs to treat pain, allergies, and coughs.

Medications that may trigger a false positive result for phencyclidine include:

  • ibuprofen (Nurofen, Advil, Motrin, Brufen, Ibu-Tab, Cuprofen, and Genpril)
  • dextromethorphan, a common ingredient in cough suppressant medication, such as Theraflu Cough Relief, Mucinex DM, and Robitussin Cough and Chest Congestion DM
  • diphenhydramine (Benadryl)
  • venlafaxine (Effexor)
  • tramadol (Ultram)
  • meperidine (Demerol, Pethidine)
  • doxylamine (Unisom)
  • imipramine (Tofranil)
  • thioridazine
  • o-desmethylvenlafaxine (Pristiq)

The most common method of phencyclidine screening is an immunoassay urine drug test. This test uses antibodies called assays to detect the presence of phencyclidine and other drugs.

Although an immunoassay test is fast, cost-effective, and easy to perform, the test may produce a false positive result.

This is because compounds from certain medications may be present in the urine sample and bind to the antibodies similarly to phencyclidine compounds.

If an IA urine test produces a positive result for phencyclidine, a person may need a follow-up urine test called a GC-MS. This is a more specific, reliable test.