Drug tests are administered by governments and by private organisations. People are most likely to be drug tested when they are applying for (or working in) certain kinds of employment, when they are suspected of drugged driving, or when they have been placed on supervision by a court. Of course, blood tests administered as part of medical diagnosis or treatment might also detect the presence of drugs.
Blood analysis is the most reliable form of drug testing. Drug tests can also be conducted on urine, bodily secretions (like sweat or saliva), and hair. Alcohol can be detected with a breath test, but no reliable breath test exists that detects the presence of any other drug.
Some tests are performed at the place where the sample is collected. Those tests are typically less accurate than tests performed in a laboratory. Other than breath tests for alcohol, only a laboratory test is usually admissible as evidence in court, although a “point of collection” test might be used for other purposes.
Unlike tests for alcohol, drug test results cannot determine whether the user is “under the influence” of the drug. With regard to most drugs, there is simply no scientific consensus about whether any specific test result establishes that the user is under the influence.
Cannabis is a good example. A drug test can detect metabolites of THC in the blood, which is strong evidence that the test subject used cannabis. But metabolites can be detected days or (in the case of a heavy user) weeks after the subject last used cannabis, long after the THC stopped affecting the subject’s brain. It simply is not possible to say anything more than “this person used cannabis at some point” on the basis of a positive test result.
“Passing” a drug test means that no drug was detected, or that the amount detected was below the threshold required for a positive result. Even if residual amounts of a drug are detected, any result below the “cut-off” for that test will be reported as passing the test. Failing the test means the result was above the “cut-off.”
How long drugs (or their metabolites) will be detectable in blood depends on many factors, including:
In light of all these factors, one person might “pass” a drug test and another might “fail” even if they both used the same drug at the same time.
Disclaimer : This article is just a summary of the subject matter being discussed and should not be regarded as a comprehensive legal advice for you to defend yourself alone. If you are charged with criminal offences, it is recommended that you seek legal assistance from criminal lawyers.