How drug test strips work

Rapid drug tests are widely used in many screening situations due to their advantage of providing results in just a few minutes. However, this method still has certain limitations.

Recently, the fact that some singers like Ngoc Son, Duy Manh, and Tuan Hung have posted images or livestreamed themselves taking rapid drug tests to prove they don't use banned substances has attracted public attention. This move has made many people curious about the mechanism of rapid drug tests and the true value of the test results.

Singer Ngoc Son proactively got tested, and Tuan Hung took a rapid test at home. Photo: FBNV.

In fact, rapid drug tests are a popular screening method due to their speed and ease of use, but they still have certain limitations.

How do rapid drug tests work?

According to the U.S. Food and Drug Administration (FDA), rapid drug tests are screening methods that detect drugs or their metabolites in the body through urine, saliva, or sometimes blood and hair samples. The mechanism of action is based on a reaction between the drug and antibodies present in the test strip.

Depending on the type of test kit, the user will collect a urine or saliva sample and drop it onto the designated area on the test strip or cup. Instructions:

• Take samples according to instructions.
• Wait for about 5-15 minutes.
• Read the result by the number of lines that appear.

When the test sample contains a drug exceeding the detection threshold, the drug will bind to antibodies, causing the T line (test line) to disappear, resulting in a positive result. If the test result suggests the possible presence of a drug, the sample should be sent to a laboratory for further testing.

Conversely, if the sample contains no drugs or the concentration is too low, the T line will appear, indicating a negative result. Additionally, the test strip always has a C (control) line to confirm the test is functioning properly. If the C line does not appear, the result is invalid.

Many modern kits can simultaneously detect multiple substances such as methamphetamine, cocaine, ketamine, morphine/opioids, MDMA, cannabis (THC), opium, amphetamine, PCP, benzodiazepines, barbiturates, methadone, ecstasy, and oxycodone.

The biggest advantage of this method is:

• Simple operation
• It gives results very quickly.
• Low cost
• Suitable for initial on-site screening.

When does a rapid drug test give a positive result?

According to the FDA, the time it takes for a drug to be detectable in the body depends on the elimination mechanism of each substance as well as the testing method used.

When a person uses drugs, these substances are absorbed into the bloodstream, then metabolized by the liver and excreted through urine, sweat, breath, or accumulated in body tissues. This process determines how long the drug can still be detected.

The time it takes for drugs to be detected in a person's body and how long that detection period lasts. Graphic: P. Mai.

The detection time also varies depending on the type of test. Blood tests usually detect the substance shortly after use, while urine tests can take from a few days to a few weeks depending on the substance. Hair tests, on the other hand, can record a longer usage history, potentially up to several months.

Furthermore, individual factors such as frequency of use, dosage, physical condition, metabolic rate, liver and kidney function, and hydration levels significantly affect the duration of drug presence in the body. This means that the same drug can yield different test results in terms of detection time for different individuals.

Are rapid drug tests absolutely accurate?

Many modern test kits are advertised by manufacturers as having sensitivity and specificity of up to 95-99% if used correctly and at the right time. However, according to the FDA, this is still only an initial screening method, not a definitive test.

In fact, rapid tests can still produce false positives or false negatives. Many factors can affect the accuracy of these tests, including (but not limited to):

• How to perform the test
• How to store test strips or urine samples
• What did that person eat or drink before the test?
• Any other prescription or over-the-counter medications that the person may have taken before the test.

For example, some cold medications, painkillers, or nerve medications can cause cross-reactions leading to false positives. Meanwhile, if a user tests too early or too late after using drugs, the concentration of the substance in the body may be below the detection threshold, resulting in a false negative. Additionally, drinking too much water or diluting the test sample can also affect the results.

In medical or forensic procedures, a positive rapid drug test result is usually re-examined using more in-depth methods such as GC-MS (gas chromatography-mass spectrometry), LC-MS/MS, or laboratory analytical tests. These are considered the most accurate standards for determining whether a person has actually used drugs.

Source: https://znews.vn/co-che-hoat-dong-cua-que-thu-ma-tuy-post1654015.html