Understanding the drug testing process and cutoffs

In workplace drug testing, the industry standard process involves two-tiered testing – an initial screen on one portion of the specimen, followed by a confirmatory test for a positive screen on a second portion of the original specimen. The initial test is designed to separate negative specimens from further consideration. The confirmatory test uses definitive methods, such as chromatography-mass spectrometry (e.g. GC-MS, LC-MS/MS), that specifically identify and quantify the drug/metabolite in the specimen.

Drug testing detects the presence of drugs and drug metabolites using cutoff levels to determine whether a specimen tests positive or negative for the use of a specific drug. Cutoff levels are expressed in nano-grams (ng) per milliliter (mL) for urine and oral fluid testing and picograms (pg) per milligram (mg) for hair testing. It is important to select a specimen type and cutoff level based on the desired window of detection and any regulatory requirements.

Only when a drug or drug metabolite is identified at a concentration equal to or above the administrative cutoff is a specimen reported by the laboratory as positive. Consequently, a negative drug test does not necessarily mean that no drug is present. While that may be the case, other possible interpretations are that a drug was present below the cutoff or the testing panel did not include the drug the individual was using.