The difference between Normal Immunoassay Drug Testing and GC-MS or Gas Chromatography and Mass Spectrometry

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The differences between Gas Chromatography and Immunoassay when performing urine drug screen testing.

Submitted: May 05, 2015

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Submitted: May 05, 2015




The difference between Normal Immunoassay Drug Testing and GC-MS or Gas Chromatography and Mass Spectrometry

by Michael Dale Sipes Jr.

Initial drug screening is performed using immunoassay. An immunoassay is a test that uses antibodies to detect the presence of drugs and other substances in urine. The initial screening process does not measure the specific amount of drug present in urine samples. It provides either a positive or negative result, indicating the presence or absence of detectable drug.

GC/MS is the abbreviation for gas chromatography/mass spectrometry, the testing methodology used to confirm presumptive positive drug screen specimens. GC/MS provides identification of the molecule(s) based on characteristic fragmentation patterns at specific retention times. GC/MS is a tandem technology, utilizing a gas chromatograph coupled to a mass spectrometer.

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, GC/MS confirmatory testing detects specific metabolites that provide identification and quantification of a specific drug.

A patient on a regiment of opiates or other drugs of abuse that is being monitored using immunoassay does not require GC/MS confirmation testing if the immunoassay testing includes the metabolites of the parent drug. If the metabolites of the parent drug are present in the specimen then the immunoassay is a complete confirmation test for a positive or negative result and requires no further testing. If a drug test using immunoassay fails to detect the metabolites of the parent drug, then a GC/MS should be ordered to confirm that the patient did, in fact, ingest the drug and was metabolized resulting in the detection of both the parent drug and it’s various metabolites.  Due to the extreme cost and labor that comes from GC/MS testing, ordering an immunoassay drug screen and requesting or adding the metabolites of the drug in question, will save the added and unneeded cost and time that results from confirmation by GC/MS testing.


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