Drug testing involves the process of testing an individual for the presence of a drug or its metabolites in the sample collected. For example, marijuana drug testing checks for the presence of THC and its metabolites. Drug testing not just involves finding the drug but also includes the establishment of drug concentration above a certain level. Cut-off level of a drug is most important in drug tests and measures the sensitivity of a drug testing device.
The concentration of a drug at or above which a drug test shows positive is called a cut-off level. Varied cut-off levels are observed for various drugs. When a cut-off level is set low, more false positive results would appear in a drug test. Similarly, when a cut-off level is set high, then more false negatives would appear. Thus, it is necessary to establish standard cut-off levels for each specific test.
The initial cut-off level for marijuana drug testing is 50ng/ml. The cut-off level for testing cocaine is 150ng/ml, opiates is 2000ng/ml, phencyclidine (PCP) is 25ng/ml, and amphetamines is 500ng/ml. Thus, when a sample tests positive by these initial cut-off levels, it is confirmed by performing further tests. For example, when a marijuana drug test gives a positive result for an individual in an initial test, the sample would be sent for confirmation. The cut-off level for confirmation are confirmatory cut-off levels. These differ from the initial cut-off levels. The confirmatory cut-off level is 15ng/ml for marijuana, 100ng/ml for cocaine, 2000ng/ml for opiates, 25ng/ml for PCP, and 250ng/ml for amphetamines.
As the positivity of a sample is based on cut-off levels, it is necessary to validate standard cut-off levels for drug tests. Many drug test kits are available which detect different types of drugs. The cut-off levels for these kits have to be standardized properly as they are used by many people at home and organizations.