Questions about False Positives in Drug Testing, page 191

False Positives Forum

This forum is intended to help those who Do NOT Abuse Drugs. We do NOT support the illegal use of any substance. If you have experience with false positives, share your experience. What would you tell someone about drug testing and false positives? Please remember that we do not give medical or legal advice.

Tobacco false positive
Date: 4/3/2012
Hello, I do not consume any form of tobacco but yet I am testing positive for cotinine. I do use a herbal toothpaste – (link removed). I am primarily a vegetarian but do consume fish. What else could be causing this false positive test for nicotine?


AskDocWeb: It could be lab error. Whenever human beings are involved, that is a possibility. However, a false positive for tobacco is more likely to come from your diet. Eating certain foods before testing can elevate levels of some substances that can cause false positives for nicotine. These foods include broccoli, cabbage, mustard, garlic, radishes, almonds and horseradish. Vegetarians can have elevated levels of a substance called thiocyanate, which can be a source of a false positive on tests for nicotine.

Thiocyanate may also come from particular work environments, especially those related to working with metal, which includes electroplaters and people who refine precious metals.

Note that nicotine patches, nicotine gum and other nicotine products used in smoking cessation programs will yield positives on tests for nicotine. These are not false positives because nicotine is actually present in the body.

Source: The American Academy of Insurance Medicine: A Comparison of Carboxyhemoglobin, Thiocyanate and Cotinine as Screening Tests

False negative results for methadone
Date: 4/3/2012
I had two false negative results for methadone. I’ve been on a low dose of methadone for years. I take it everyday. my pm decides to test me. I had 2 blood serum test come back clean. I take several other meds and have always had problems metabilizing meds fast. the other meds i take are cymbalta, lyrica, ambien, zyertec, vit d and imitres shots as needed for migraines. i’m loosing a doc over this and need to know for my own piece of mind why it’s coming back clean??? Thanks,


AskDocWeb: You are not the first person to complain about this and it happens for drugs other than just methadone. In 2004 as many as 44% of patients tested by the VA for medication compliance were found negative for opiates despite the patients being prescribed opioid medication. The VA concluded that the immunoassay test used was not sensitive enough. In other words the cutoff level was too high to detect the presence of opioids that the vets were taking.

There are also a number of other reasons for testing false negative:

  • The drug was not taken as prescribed.
  • Drinking an unusual amount of water may also dilute the test sample. A diluted sample may not have the concentration level that is needed to show that you are taking the drug.
  • Some people metabolize drugs exceptionally fast and if the last dose was taken too long ago it might not show up.
  • A small percentage of drug tests are faulty.
  • If the drug you are taking has a very short half-life and if the last dose was taken too long ago it might not show up.

One solution is to request a blood test so that they can get a more accurate result.

When a urine test comes back clean (false negative) and fails to show the drugs that are expected to be there, it is not unusual for a doctor to stop treating that patient. A clean test in that situation is known to be one of the signs that patients are selling their medications on the street. Doctors who continue to prescribe drugs to those patients can lose their license and/or face prison time. Is it any wonder that doctors dismiss patients in that situation?

Naproxen (Aleve)
Date: 4/4/2012
Hi! I have prescription naproxen for carpal tunnel syndrome in my hands and I came up positive for THC on a urinalysis. Could the naproxen have caused the false positive? If so, would the lab be able to distinguish the two? What ng cut-off point would be safely able to distinguish between THC and naproxen? Thank you so much for your help. Have a great day.


AskDocWeb: We found three sources that say Naproxen might test false positive for THC or marijuana (See references 3, 16, and 18). The confirmation lab will be able to distinguish the difference but there is no cut-off level for that answer. It will be a simple negative for the presence of THC.

Blood test?
Date: 4/4/2012
Would a blood test prove that i have not used marijuana since february 14 of 2012?


AskDocWeb: A blood test may show whether or not the metabolites for marijuana are in your system but it can not tell how long ago any drug was discontinued.

Problem with storing medications
Date: 4/4/2012
My son tested positive for meth amphedimine on a urine drug screen. He has not done any illicit drugs. He has been taking Motrin, AlkaSeltzer Cold Plus, benadryl, OTC robitussin, Clindamycine (stored in A bottle with Topamax, and Phentermine, both mine, But I removed All those pills from the bottle). Could this cause a False Positive?


AskDocWeb: Several of the medication you list could result in a false positive on a urine drug test. However, in order to prove that his test result was a false positive the original sample must undergo additional testing with a confirmation test.

Please note that storing multiple medications in the same bottle increases the chances of accidentally taking the wrong medication, especially when they are for more than one person.

Cholestasis of pregnancy?
Date: 4/4/2012
I recently gave birth, but was diagnosed with cholestasis of pregnancy a week before delivery. When I went for my pre-op, it came back positive for marijuana use. I was completely devastated as I have NEVER used drugs, much less while I was pregnant. I retested the following night before giving birth and it was negative. Can cholestasis cause a false positive marijuana test? How can I get this cleared off my medical record??


AskDocWeb: Cholestasis is a condition in which little or no bile is secreted or the flow of bile into the digestive tract is obstructed. We have not heard of that condition causing any kind of false positive for drugs of abuse. As mentioned in the post above, the original sample would have to be sent out for confirmation testing and come back negative before your record could be clarified.

Drug test 6633n sap 10-50
Date: 4/5/2012
I went to the doctor last Friday which was 7 days ago and was prescribed suboxone. That was the last day I did an oxycontin. I had to take a drug test today so last night I went and got a 12 panel drug test and was positive for oxycontin but I haven’t touched them in over a week. Would the suboxones make a false positive for oxycontin? And does oxycotin show up on a 6633n sap 10-50? Thanks for your help.


AskDocWeb: The 6633n sap 10-50 is a Quest Labs designation for a standard 10 panel urine test. This test has a 50 ng per ml cut-off level for the initial screening. If the urine tests presumptive positive for anything it undergoes additional testing by Gas Chromatography/Mass Spectrometry to rule out false positives.

According to the American Association for Clinical Chemistry, suboxone may test false positive for oxycodone (oxycontin) on urine drug tests.

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