The standard in toxicology testing is to utilize cut-off levels. Cut-off levels are recommended by the U.S. Department of Health and Human Services (HHS). Drug cut-off levels are the minimum concentrations of drugs or metabolites that must be present in specimens, before labs will report the drug testing results as positive.
All specimens are screened at the laboratory and specimens with a positive result after screening go to a second testing methodology to confirm the screening result. The universally accepted confirmation test is Gas Chromatography Mass Spectrometry (GC/MS).
Simply stated, screening and confirmation testing are performed using different testing methodologies that precipitate different cut-off levels. The immunoassay tests used to perform initial drug screening are designed to detect a wide range of chemically similar compounds that react with the antibodies which are at the core of the chemistry making up the tests. In contrast, gas chromatography/mass spectrometry (GC/MS) confirmatory testing detects specific metabolites that provide identification and quantification of a specific drug.
Different drugs have different cut-off levels based on standards set by the Federal government. The levels are set so that the final verified drug test results can absolutely without any doubt provide information that the individual with a positive drug test had ingested the particular drug prior to the drug test. The gas chromatography/mass spectrometry (GC/MS) cut off levels are designed to rule our claims of second hand smoke or potential false positives due to over the counter medications.
Example
A test for marijuana shows a level of 80 ng/mL for marijuana, the screening cut-off level is 50 ng/ml so this goes to confirmation testing. The confirmation test shows a level of 60 ng/mL for THC – this confirms the presence of the metabolite of THC in the specimen, and the test reported as positive. If the confirmation level is at 10 ng/ML the test would be reported as negative – the confirmation cut-off level for a marijuana test for a urine specimen is 15 ng/ML.
