Can a Drug Test Lead to a False Positive?
Medically reviewed on May 26, 2016 by L. Anderson, PharmD.
Have you been asked to have a drug test? Maybe this caught you off guard. You probably have many questions and concerns. The most important take-away: You should confirm that GC-MS confirmatory testing is performed on any positive result received by the lab doing your testing. Here are more details.
What Drugs Are Tested?
Drug tests are typically done on urine, but other types of specimens are becoming common, such as hair, saliva or blood. The most commonly screened substances, and also required by Federal workplace guidelines, include at least these five categories of drugs:
Additional categories, as noted by SAMHSA, may include:
- Designer drugs like Bath Salts, Spice, or Ecstasy (MDMA)
- Benzodiazepines, like diazepam (Valium) or alprazolam (Xanax)
- Ethanol (alcohol)
- Prescription opioids like hydrocodone, oxycodone, methadone
Consequences of False-Positives
Drug screening results can negatively affect many circumstances; therefore, accuracy is of the utmost importance. A worry for anyone undergoing drug testing - whether it be a urine, hair, saliva or blood test - is the possibility of a false positive result. The increased use of onsite, workplace random drug testing and home-testing kits emphasize the need for reliable, confirmatory testing. Negative consequences of false-positive drug testing can include loss of employment, jail time, exclusion from competitive sports, loss of privileges in a probation setting, or even inappropriate medical care.1
When initial screening drugs tests (called immunoassays) result in positive results, a second confirmatory (Gas Chromatography Mass Spectrometry or GC-MS) test - which should always be done on positive results - greatly lessens the chance of a false positive, almost reducing the risk to zero.2 However, GS-MS can also lead to a false negative if the GC-MS column is not designed to identify all potential compounds.
The immunoassay allows quick, large scale screenings with minimal cost. When urine or other specimens are collected, they are split into two samples, and one should be saved for confirmatory testing. For instances where an initial drug screen returns a positive result, the GC-MS test is run on this additional saved portion to confirm or deny results. GC-MS is considered the “gold-standard”.
In some rare cases, a drug test may report the presence of illicit or prescription drugs in the immunoassay, although the person has not used these drugs. While this is not common, no test is 100% accurate. It is important that if you are undergoing drug testing, that you give a complete and accurate history of all prescription, OTC, and vitamin/dietary supplement/herbal drug use prior to the time of the sample collection. The results of drug testing should remain confidential and kept separate from the regular employee work file.
False-positive drug test results have been reported in multiple drug classes. Some of the more common classes include:
- Certain OTC products
The chart below identifies substances reported to lead to a false-positive result based on the initial immunoassay testing. Remember, any positive drug test should always be confirmed with a secondary test such as GC/MS that detects and provides identification and levels of a specific compound.
Substances Reported to Lead to False Positive Urine Drug Screens3-8
|Substances that may interfere with drug testing:||Reported false positive result:|
|Benzphetamine (Didrex, Regimex)||Amphetamines1|
|Chlorpromazine (Thorazine)||Amphetamines/methamphetamines7, Methadone7|
|Coca leaf tea||Cocaine1|
|Cocaine anesthetics, topical||Cocaine1|
|Cyclobenzaprine (Flexeril)||Tricyclic antidepressants1|
|Dextromethorphan (Robitussin Cough, Delsym) - see notes below||Phencyclidine (PCP)1,7, Opiates1|
|Diphenhydramine, Doxylamine (antihistamines)||Methadone7, Opiates1, Phencyclidine (PCP)1, Tricyclic antidepressants1|
|Ephedrine nasal inhaler||Amphetamines/methamphetamines1,3|
|Hemp oil (may be used as a nutritional supplement)||Marijuana (cannabinoids), tetrahydrocannabinol (THC)1,5|
|Ibuprofen, naproxen, tolmetin (NSAIDs) - see notes below||Marijuana (cannabinoids), barbiturates, benzodiazepines1; Phencyclidine (PCP)1,7|
|Pantoprazole (Protonix), possibly other Proton Pump Inhibitors (PPIs)||Tetrahydrocannabinol (THC)1,4,5|
|Poppy seeds - see notes below||Opiates/morphine1,3,7|
|Quetiapine (Seroquel)||Methadone7, Tricyclic antidepressants1|
|Quinolone antibiotic (ofloxacin, gatifloxacin)||Phencyclidine (PCP)7, Opiates1|
|Selegiline (Eldepryl, Zelapar)||Amphetamines1,8|
|Sertraline (Zoloft) - see notes below||Benzodiazepines1,6,7|
|Thioridazine (Thorazine)||Methadone1,7, Phencyclidine (PCP)1|
|Trimethobenzamide (Tigan, Ticon, Benzacot)||Amphetamines1|
|Venlafaxine (Effexor)||Phencyclidine (PCP)1,7|
|Verapamil (Calan)||Methadone7, Other opiates1|
Poppy seeds, often found on bagels, rolls, and pastries have long been used as a defense against positive opiate test results encountered on a urine drug screen. It is known that poppy seeds do contain opiates - specifically morphine and codeine; however, content varies greatly depending upon seed source and processing. Food processing may lower the opiate levels in poppy seeds.
In a 2015 study published in the Journal of Analytical Toxicology,9 researchers determined opiate concentrations in opiate-free volunteers from 15 minutes to 20 hours after consumption of raw poppy seeds (15 grams) and prepared poppy seeds on a roll. Concentrations, both 300 and 1,200 ng/mL were evaluated in urine samples. Oral fluid concentration cutoffs are set at 30 ng/mL.
Using the 2,000 ng/mL cutoff for urine testing, morphine was not detected in urine from seeds on rolls; raw poppy seed consumption resulted in a detectable level in urine during the first 6 hours after consumption. At 20 hours, no morphine was detected for rolls or raw seeds in urine testing.
With the 300 ng/mL cutoff for urine testing, morphine was detected after rolls in 50 percent of samples at 20 hours, and in 100 percent of raw poppy seeds after 20 hours. With oral fluid testing, morphine fell below the 30 ng/mL detection limit with rolls after 30 minutes and from raw seeds at one hour.
Oral testing and the higher cutoff level of 2,000 ng/mL may help to reduce the number of false positives due to poppy seed consumption. However, the public typically eats rolls or pastries and would have no reason or desire to consume unpalatable raw poppy seeds where morphine levels are detected even after 20 hours at the 300 ng/mL limit.9
Oral and urine drug testing can typically detect opiates up to 48 to 72 hours after use.
In November 1998, federal authorities who mandate drug testing for federal employees raised the required morphine cutoff concentration from 300 to 2,000 ng/mL to reduce the number of opiate false positives due to poppy seed consumption.
Over-the-counter cough and cold products such as the cough suppressant dextromethorphan have been reported to lead to a false positive result for PCP and possibly opiates. Dextromethorphan is chemically related to levorphanol, a narcotic analgesic. The body metabolizes codeine to morphine and both substances may be found upon testing. Confirmatory testing can distinguish between the products.
NSAIDs, such as ibuprofen, have often been implicated in causing false positives but most specimens are corrected with an alternate screening methodology if this is of concern. NSAIDs are easily available OTC in the US and are widely used. In one report, authors concluded that acute or chronic use of ibuprofen or chronic use of naproxen were not regularly associated with false-positives but they do recommend secondary testing if needed.
According to one report, doses of sertraline (Zoloft) exceeding 150 mg/day could lead to false positive benzodiazepine urine drug screens. Nasky and colleagues10 reviewed 522 positive results for drug screens for benzodiazepines while taking sertraline but negative with GC-MS. They found 26 of 98 records to be identified as false positives. The package insert for sertraline now warns about this possibility under the ‘Laboratory Tests’ section.
Novocaine use as a defense for cocaine abuse is not valid. If benzoylecgonine, the main metabolite of cocaine is detected, the subject cannot claim that the result is a false positive due to Novocaine administration, or any other "-caine" drug. Benzoylecgonine is only found in nature as a metabolite of cocaine, and there would be no other valid reason for its presence in a drug screen.2 As previously mentioned, confirmatory testing with GC-MS will identify individual drugs or metabolites in a sample, and almost eliminate the chance for a false positive result.
Passive marijuana smoke
The argument of inhalation of "passive" smoke from being in a room with people smoking marijuana is not valid, as the cut-off concentrations for lab analysis are set well above that which might occur for passive inhalation. All of these variables, and others, are looked at in the lab analysis, keeping one step ahead of those that attempt to foil drug tests.
While it is difficult to tamper with a specimen, attempts have been made to alter samples, most often urine. Abnormalities in the urine screen may indicate that results may be a false negative or that there was deliberate adulteration of the sample.
A low creatinine lab value can indicate that a urine sample was tampered with - either the subject diluted their urine by consuming excessive water just prior to testing, or water was added to the urine sample. Creatinine levels are often used in conjunction with specific gravity to determine if samples have been diluted. To help avoid this problem, the testing lab may color the water in their toilet to prevent the sample from being diluted with toilet water.
Those attempting to foil the drug screen process may try to add certain enzymes to the urine sample to affect stability, but this often changes the pH, which is also tested. The most common adulterants used include certain oxidizing agents – such as nitrites, glutaraldehyde, chromates and halogens like bleach and iodine. Testing for adulterants in addition to drugs may be requested by the person requiring the drug screen. The temperature of urine is also measured, to help prevent use of a substituted urine sample brought in from the outside.
To join others who may have questions, concerns or want to voice opinions about illicit and prescription drug testing, false positives, and other related issues, visit the Drug Testing Support Group on Drugs.com
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- Moeller K, Lee K, Kissack J. Urine Drug Screening: Practical Guide for Clinicians. Mayo Clin Proc. 2008;83:66-76.
- Doering, PL, Boothbay LA. Drug Testing in the Workplace: What the pharmacist should know. Drug Topics (Modern Medicine) 2003;147:63.
- Oliff H. Drug Tests: Don't Fall Victim to a "False-Positive". Accessed May 21, 2016 at http://health.cvs.com/GetContent.aspx?token=f75979d3-9c7c-4b16-af56-3e122a3f19e3&chunkiid=13837
- Protonix Package Insert. March 2012. Accessed May 22, 2016 at https://www.drugs.com/pro/protonix.html
- Felton D, et al. 13-year-old girl with recurrent, episodic, persistent vomiting: out of the pot and into the fire. Pediatrics. 2015 Apr;135(4):e1060-3. doi: 10.1542/peds.2014-2116. Epub 2015 Mar Accessed May 21, 2016 at http://www.ncbi.nlm.nih.gov/pubmed/25733759
- Sertraline. Drugs.com. Accessed May 21, 2016 at https://www.drugs.com/pro/sertraline.html
- Brahm N, Yeager L, Fox M, et al. Commonly prescribed medications and potential false-positive urine drug screens. Am J Health-Syst Pharm. 2010;67:1344-50.
- Cody, J. D. Metabolic Precursors to Amphetamine and Methamphetamine. Forensic Science Review 1993:5(2):109–27.
- Quest Diagnostics. Challenging the Poppy Seed Defense. White Paper. “Concentrations of Morphine and Codeine in Paired Oral Fluid and Urine Specimens Following Ingestion of a Poppy Seed Roll and Raw Poppy Seeds. Accessed May 23, 2016 at http://blog.employersolutions.com/challenging-the-poppy-seed-defense
- Nasky KM, Cowan GL, Knittel DR. False-positive urine screening for benzodiazepines: an association with sertraline? A two-year retrospective chart analysis. Psychiatry. 2009; 6:36-9.