How do False Positives Occur on Drug Tests?

Most people taking a drug test don’t have to worry if they haven’t been using, but some instances are an exception. False positives are a relatively rare phenomenon in urine screening, blood tests, and other types of drug tests in which an individual’s diet, medication, or a fluke in the test can create a false positive. Here, an individual who has not been using is flagged as having levels of a substance in their blood indicative of drug use.

If you or a loved one has had a positive result on a drug test and you think it might be or know it is mistaken, learning about the ways false positives occur on drug tests can help you fight results and ask for a second opinion. Importantly, if the test is for a loved one, it’s crucial that you discuss with them before approaching their workplace or doctor to ensure they are not actually using. At the end of the day, false positives are rare and chances are if a test says positive, it’s more likely to be true than false. Of course, there are always exceptions.

How Do False Positives Occur on Drug Tests?

Most drug tests are conducted for work or legal reasons, typically using a urine, hair, saliva, or blood test. These samples are collected onsite and then typically processed privately at a lab using a series of technology. Here, nearly all drug tests begin with a process known as immunoassays, or drug screening. These tests allow for large-scale screening at a relatively low cost, making it more affordable for workplaces to test their entire organization, individuals to request personal tests, and government to use tests as verification.

Drug tests typically use the following process:

  1. A biological sample is collected from the patient
  2. The sample is split into two, an initial test and a confirmation test
  3. An immunoassay is run to identify if the individual is using
  4. In case of a positive, a second test (GC-M) is run to verify results

In most cases, false positives occur at the immunoassay stage. These drug tests are relatively fast and false positives are rare. However, anyone going for an immunoassay should alert testers to any prescription drugs, vitamins or dietary supplements, and herbal drug use to avoid a false positive.

False positives occur during Enzyme Immunoassay (ELISA) because the test either fails to differentiate similarly structured molecules from drug molecules, or you have taken or consumed items that contain some of the same enzymes and molecules as the drug you are being tested for. Here, poppy seeds are a commonly-known culprit of false-positives.  

What Happens When a False Positive Occurs?

It is common practice (although not always) to run a secondary confirmatory drug test on any positive results. The standard drug test for this secondary confirmation is known as Gas Chromatography Mass Spectrometry (GC-MS). Running a GC-MS on an immunoassay reduces the chances of a false positive to almost zero, and most false-positives are acquitted at this stage. In fact, GS-MS tests are more likely to result in a false-negative if the GC-MS is not broad enough.

If you are not given this secondary screening, you may have the right to request one from your employer or from the law. Be sure to ask about your rights regarding a secondary screening.

What Items Might Cause a False Positive?

While there are many substances that can cause a false positive, NSAIDS such as Ibuprofen, fluoroquinolone antibiotics, and Vicks Inhaler are the most commonly reported. Therefore, if you are going into a drug test and have frequently used an over-the-counter painkiller, antibiotics, or inhalers, you should inform the lab technician during the test.

  • Antibiotics
  • Amantadine (Symmetrel) – Amphetamines
  • Beef – Anabolic steroids
  • Benzphetamine (Didrex, Regimex) – Amphetamines
  • Brompheniramine – amphetamines
  • Bupropion (Wellbutrin) – amphetamines and methamphetamine
  • Chlorpromazine (Thorazine) – Amphetamines/methamphetamines, Methadone
  • Cocoa Leaf Tea – Cocaine
  • Dextromethorphan (Robitussin Cough, Delsym) – Opiates, Phencyclidine (PCP)
  • Diphenhydramine, Doxylamine (antihistamines) – Methadone, Opiates, Phencyclidine, Tricyclic antidepressants
  • Diet pills – Amphetamines
  • Desirapram – Amphetamines
  • Hemp Oil – Marijuana, CBD, THC
  • Labetalol – Amphetamines
  • Methylphenidate – Amphetamines
  • Nasal inhalers – Amphetamines
  • NSAIDs (Ibuprofen, Advil, paracetamol, naproxen, etc.) – Marijuana (cannabinoids), barbiturates, benzodiazepines1; Phencyclidine (PCP)
  • Oxaprozin – Benzodiazepines
  • Phenylpropanolamine – Amphetamines
  • Phentermine – Amphetamines
  • Poppy seeds – Opiates
  • Promethazine – Amphetamines
  • Pseudoephedrine – Amphetamines
  • Quetiapine (Seroquel) – Methadone, Tricyclic antidepressants
  • Ranitidine (Zantac) – Amphetamines/methamphetamines
  • Selegiline (Eldepryl, Zelapar) – Amphetamines
  • Sertraline (Zoloft) – Benzodiazepines
  • Venlafaxine (Effexor) – PCP
  • Verapamil (Calan) – Methadone/ Opiates

This non-comprehensive list will give you some idea of what to look for in your diet or prescription medication in case you do receive a false positive. Your best solution is to simply bring along a list of any prescriptions you have and ensure they are noted down with your blood test to avoid a false positive, you might also want to educate yourself on different drugs (such as reading informational articles like this What is Angel Dust and What are the Side Effects – Health Street) that could be tested for to ensure you have the knowledge to confirm you are not in fact under the influence of drugs that have come back as a false positive. However, in the case that you do receive a false positive, the follow-up confirmation testing will most likely vindicate you, as chances of a secondary false positive are all but zero.

In addition, most labs have standard practices to avoid identifying known-substances as a false positive in follow-up GC-MS testing. For example, most labs can differentiate between opiates in the blood because of poppy seed consumption and opiates because of opiate usage based on cycle count and volume in the blood which holds true unless you were to eat more than a pound of poppy seeds over the course of a few days before your drug test.

What if your GC-MS Test Comes Up Positive?

Drug testing has become increasingly more accurate over the past few years, with estimated false-positives dropping from 10-15% to less than 1.75%. In fact, most false positives include other illicit drugs. For example, 50-90% of all false-positive amphetamine tests eventually showed MDMA in the blood. As a result, modern GC-MS tests have almost a zero chance of a false positive. If you haven’t taken any drugs and have come up positive on a GC-MS drug test, your best option is to request a second opinion and to submit a full list of the prescription medications you may be on. This situation is unlikely and typically relates to human rather than instrument error but is possible.

If a family member has come up positive you should question their habits and potential drug use. Addicts will often lie to cover up their abuse. While it’s important to show trust, chances are that if they have come up positive after a GC-MS, they are using or have used in the past. If their drug test report aligns with recent (or even old) changes in behavior, reckless behavior, substance abuse, isolation, lack of money, or physical signs of drug abuse or addiction, it may be important to seek out help.

If you or a loved one is facing a drug test, it’s important to be honest with yourself and with others. You may lose your job, academic position, or face shame in your community for coming up positive on a drug test, but there is no shame in asking for help. Most workplaces offer programs to protect and support individuals seeking rehabilitation. If you do so, you and your job are protected by law under the Family and Medical Leave Act and under the Patient Protection and Affordable Care Act. If you seek out treatment before your drug test, these regulations will protect you unless your employer can prove you used at work.

If you don’t use and you have a positive result on your test, it’s simple to ask for a confirmation test. Chances are, your employer is required to do so by local or federal law so you might not have to ask. If both tests come up positive, there are a variety of ways you can request a second opinion, submit additional evidence to the contrary, and work to change the result of the test. Unfortunately, you won’t likely be able to go in for follow-up testing for many drugs, unless you are using hair or nail samples.

If you or your loved-one needs help, please contact South Coast Counselling at 1-844-330-0096 and speak with one of our experienced treatment advisors today in complete confidence.