It’s not unusual for us to hear from customers new to drug testing who think a “negative” drug test means that a specimen contains zero drugs.  In fact, a negative drug screen does not necessarily mean zero amount of the drug in the specimen.  Industry practice defines a negative drug screen as a specimen that does not contain drugs at, or above, a specified concentration of drug.  The specified concentration of drug is called the cut-off level.  A specimen that contains drug at or above the cut-off level is a presumptive positive.  A specimen can contain a small concentration of drug that is below the cut-off level and still be correctly classified as a negative drug screen. 

The U.S. Dept. of Health and Human Services establishes guidelines for drug testing Federal employees.  Dept. of Transportation drug test regulations reference the HHS guidelines, as do many private-sector employers.  While the HHS cut-off levels are intended for use by laboratories, many instant drug screen kits use the same cut-off levels.  Here are the current HHS cut-off levels for drugs of abuse.  Cut-off concentrations are expressed in nanograms per milliliter.