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PostPosted: Tue Sep 28, 2010 7:38 pm 
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Length of time a drug may show positive on a urine screen:

A number of inquiries concern the length of time a drug may register positive on a urine test. There are several legitimate reasons to know this, not the least of which is to avoid problems in what may be an already complicated situation. The following information is from a major manufacturer of drug test kits and equipment. It it the most authoritative and comprehensive source I have been able to find. Be aware that the following duration of drug persistence applies to established minimum cut off points not an absolute negative result. To be safe for an absolute negative result, double the times given below.

Not all tests include all drugs. It depends on the panel of drugs tested for in that particular test. So when someone says “Subutex doesn't show up on a drug test” what they are really saying is the test they took didn't screen for it. It is possible for any substance to be detected in urine if it is included in the screen. This is the reason for confusion about which drugs show up and which don't. Different tests for different purposes screen for different drugs. The following information is for customers of the test manufacturing company – laboratories, security services, government agencies - who purchase the tests so I believe the information is reliable.

Typically a drug screen has two phases. The first is a low-cost test that screens for a specific panel of drugs. This phase is the one when false positives can appear. If a positive result is shown on the first test a second test is performed on the same sample. This test is more expensive and is not performed if the first test shows all negative. The second test is a chromatography test. It can absolutely identify the presence of any substance that it tests for. This is the source of confusion about “false positives”. The first phase of testing can produce a false positive, the second will not.

Drug Retention Times for standard cut off levels (double for absolute negative results):

Amphetamines: 2 - 6 days
Barbiturates: 3 - 8 days
Benzodiazepines: 2 - 14 days
Cannabis (THC): 14 - 28 days
Cocaine: 2 - 5 days
Methadone: 2 - 7 days
Opiates: 2 - 5 days
PCP: 2 - 6 days
Subutex: 3 – 4 Days

Considerations of age (metabolism), weight, diet, purity of the drug, and, frequency and duration of use may affect these estimates.

Test Cut-off levels:

The cut-off level is the level at which a diagnostic test determines whether a sample is positive or negative. The level at which a sample becomes positive varies from drug to drug but is based upon the research and guidance of SAMHSA in the USA (the Substance Abuse and Mental Health Services Administration) and by the predecessor of SAMHSA (the National Institute on Drug Abuse).

The reason for a standard cut-off level is for all laboratories to test at the same level, so that wherever a test is conducted, the patient will be assessed by the same criteria and that results from one region are comparable with those of another.
The cut-off levels are set at a point that is high enough to rule out passive exposure and low enough to ensure that a patient must have abstained from the drug to test negative. It's a little like adjusting a metal detector to ignore belt buckles but detect weapons.

Masks:

A mask is a mixture of chemicals that neutralize the compounds that are tested for on the first screen. If purchased from a reliable supplier they are effective. However they have three drawbacks. The first is if one is taking Protonix (acid reducer) for example, it will show a false positive for cannabis on the first test even though the cannabis didn't. The mask is designed to conceal the presence of cannabis, not Protonix which returns a false positive. That will escalate the test to the second level – chromatography. That second test will register the presence of cannabis regardless of the mask. Masks are only effective if no positives escalate the test to the second level.

The second drawback of masks is that they only mask the specific substance for which they are designed. Usually these are cannabis, opiates, cocaine, and methamphetamine. Substances that are part of the screen that it is not specifically designed to neutralize will register on the test therefore escalating it to the next level. Considering the ethical questions (third drawback), combined with the risk of false positives which take the testing to the next level, it is best to avoid masks and, if desirable, rely on eliminating the drugs from the system prior to testing if possible. Taking the higher ethical road is always a better choice for peace of mind and it's a good habit to develop.

Accurate knowledge gives real power over one's decisions. It is a tool to make better choices. Like any tool it is only as good as it's reliability. Real knowledge is not guesses or speculation – it is a good foundation for good choices.


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Fond Du Lac Psychiatry
Dr. Jeffrey Junig, M.D., Ph.D.

  • Board Certified Psychiatrist
  • Asst Clinical Professor, Medical College of Wisconsin

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