The cut-off value is the concentration of a substance above which a result is considered "positive". So if the concentration of a drug of abuse in urine is below the pre-defined cut-off the result is considered "negative".
For medical drugs the terms therapeutic threshold and therapeutic range are used. If the concentration of a drug is within this range one can expect the desired actions of the medication and side-effects can be avoided. If the therapeutic threshold is not reached as a result of a too low dose therapeutic effects will fail to appear. High doses in contrast with drug concentrations exceeding the therapeutic range will cause side-effect and toxic symptoms.
Threshold values have also been recommended for pesticides, heavy metals or organic substances which are used at place of work e.g. ADI values (acceptable daily intake), which should not be exceeded if health of the workers should not be endangered.
Depending on the industrial sector public authorities have defined threshold values for toxic substances in urine and blood. Due to continously growing and new scientific knowledge it is not unusual that cut-off and threshold values are adjusted or corrected from time to time.