When the test result shows negative, can we be sure that the person did not take drugs?

No, several factors may cause a negative test result, even if the person is using drugs. The most common factor is the adulteration of the sample (e.g, addition of water to dilution the sample) in an effort to lower the drug concentration below the cut-off value.  If there is suspicion that person is taking drugs, you should test again, test for other types of drugs, and talk to your doctor for advice.

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