Drug Test False Positive False Negative

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Published on Oct 17th, 2016

In drug screening, a “false positive” means that a sample is falsely reported as positive even if it is actually drug-free. Conversely, a “false negative” means that a sample is reported negative despite the donor actually having taken drugs prior to the test.

A drug test false positive (or a false positive drug test result) is an erroneous test result that says a patient or test subject possesses a significant amount of drug metabolites in the body. Meanwhile, a drug test false negative indicates that the test subject does not have a significant level of active drug components in the body even though the person has taken the drug in reality.

Drug test false positives may stem from several possible reasons, some of which are the following:

  • Incorrect testing methods. When a positive result arises in initial screening tests, confirmatory tests such as the GC/MS method should be conducted in order to remove any likelihood of getting a false positive to nil.
  • Very low drug test cut-off level. If the drug testing laboratory uses a minimum threshold value for drug confirmation that is too low, test subjects who underwent incidental exposure to drug components may get a positive result. For example, secondhand smoke from marijuana may render the results positive if the cut-off level is too low.
  • Cross-reactivity of compounds similar to drug components. It is possible that other compounds similar to active drug metabolites may be detected, such as poppy seeds incorrectly assumed to be opiates.

On the other hand, false negatives may result from the following factors:

  • Sample adulteration. Dilution of a urine sample with water may reduce the concentration of a specific drug metabolite. However, modern testing methods have already considered this scenario, and are using additional testing techniques to determine whether the sample has been tampered.
  • Very high drug test cut-off level. Setting a high cut-off level may lead to non-detection of samples that contain a significant concentration of a particular drug.

For any test result – whether it’s positive or negative – initial screening tests must be followed by confirmatory testing. This should eliminate the occurrence of a false positive or false negative result. The best way to prevent a false positive result is to not take the drug at all.

How Do Drug Testing False Positives Occur?

Many factors can cause a false positive to be reported, namely:

  • Improper laboratory procedures – Laboratories should follow very strict, quality-controlled procedures when handling samples and performing chemical analysis on said samples as making procedural short-cuts or not observing best practices can possibly compromise samples such that they result in erroneous results.
  • Mixing-up samples – At the collection site, mixing up of samples can happen if the collectors are negligent enough not to properly seal and label the samples immediately upon receipt.
  • Passive inhalation/accidental exposure – Some people unknowingly become exposed to minute amounts of substances by simply being around people who use them, like sitting next to someone who’s smoking marijuana, or eating poppy-seed laden bagels or cakes. They end up testing positive for THC or morphine.
  • Very low cut-off levels – Some drug testing laboratories use very low cut-off levels that incidental/accidental exposure to drugs may result to a false positive result. For example, exposure to second-hand marijuana smoke or consuming poppy seeds can manifest as a false-positive for THC or morphine for someone who goes in for a drug test at a lab with very low cut-off levels.
  • Cross-reactants – Some compounds have chemical structures that are similar to a targeted drug or its metabolites. They are then misidentified and result to a test being reported as positive. Many substances can cause cross-reactivity on an immunoassay screen:
  • Antibiotics
  • Antidepressants
  • Antihistamines
  • Antipsychotics
  • Cold Remedies/Decongestants
  • DHEA Hormone
  • Ibuprofen
  • Melanin
  • OTC drugs
  • Pain relievers
  • Poppy seeds

If a positive test result is reported by a drug screening and the donor admits to using drugs, no further tests need to be done. However, if a result comes back positive and the donor denies having used drugs, a confirmatory test is required, usually Gas Chromatography Mass Spectrometry (GC/MS).

I Received a False Positive Drug Test Result: NOW WHAT?

How Does a False Negative Drug Test Happen?

Some factors can cause a false negative to be reported, namely:

  • Adulterated specimens – There are many creative ways to tamper with samples, usually urine. Urine specimen adulteration is done by diluting, enhancing or substituting. Many adulterants are openly sold on the internet and while a lot of them don’t actually work, some manage to return false negative results.
  • Very high cut-off levels – Laboratories that use very high drug cut-off levels end up returning mostly false negative drug test results because even if a sample does contain illicit substances or their metabolites, the concentration levels still do not reach the very high cut-off set by the lab, and the test comes back negative.

It is the false positive tests that make the most noise as indignant donors naturally want to prove their innocence. However, it is actually the false negatives that pose the greater safety risks, because despite having consumed illicit substances, no one who gets a false negative drug test will complain about the result.

For any drug test result – whether it’s positive or negative – initial screening tests must be followed by confirmatory testing. This should eliminate the occurrence of a false positive or false negative result. The best way to prevent a false positives on drug test results is to not take the drug at all.

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