Differences Between Drug Testing Methods
Assay vs. GC/MS (Gas Chromatography-Mass Spectrometry) : The Basics
Enzyme Immunoassay
Also known as an assay test, this method detects the presence or activity of an individual drug or drug family in saliva, urine, and blood. This methodology is most commonly used for at-home drug testing kits or for pre-screening at a laboratory.
Assay testing works by using a protein that binds to the metabolites of any of the following:
- Alcohol
- Amphetamines
- Barbiturates
- Cocaine
- Fentanyl
- Heroin
- Nicotine
- Opioids
- THC (Marijuana)
- And more
Once the protein binds to any of the substances above, the test strip color will change.
For example, when you take a marijuana assay test, the strip next to T will appear (even faintly) if the specimen is negative for THC.
Presently, urine remains the specimen of choice because it carries a higher concentration of drugs and metabolites than blood. It also has a longer window of detection compared to saliva.
Assay tests are highly sensitive, meaning they can detect the presence of the drug if the levels are above the cutoff concentration. That’s because the test protein will only bind to a specific substance - for example, THC - and nothing else.
According to the SAMHSA Clinical Drug Testing in Primary Care manual, assay tests can identify 95%-98% of true negative and 100% of true positive results.
Gas Chromatography/Mass Spectrometry
GC/MS is a type of confirmatory testing that helps determine the accuracy of assay results.
GC/MS identifies the presence of certain drugs in saliva, urine, blood, as well as hair. It does so through the following methods:
- Gas-liquid chromatography, which identifies a substance’s vapor components and how it interacts with solid surfaces
- Mass spectrometry, which identifies the unique mass spectrum or the ratio of ions present in a drug
Both processes help ensure that substances in the sample are identified clearly. This makes GC/MS a highly specific test, meaning it can’t detect a component if it’s below cutoff levels.
Assay and GC/MS Compared
Both tests have their own pros and cons, so let’s compare them up close:
|
Assay |
GC/MS |
|
|
Purpose |
Preliminary, qualitative testing |
Confirmatory, quantitative testing |
|
Substances That Can Be Detected |
Alcohol, Amphetamines, Barbiturates, Cocaine, Fentanyl, Heroin, Nicotine, Opioids, and THC (Cannabis) |
Same + other abused drugs (illicit or prescription) |
|
Testing Procedure |
Collect urine, dip the strip into the sample, and wait for the results |
Collect urine and bring/ship it to the laboratory |
|
Ease of Use |
Can be done by the test-taker at home |
Must be processed through complex machines by a trained professional |
|
Time to Get Results |
5-30 minutes |
3-5 days |
|
Substance Detection |
Detects metabolites according to set cutoffs (i.e., a standard kit can check for THC levels above 50 ng/ml) |
Identifies and detects the exact presence of metabolites due to lower threshold levels (i.e., the cutoff for GC/MS THC testing is 15 ng/ml) |
|
Adulterant Detection |
Adulterant strips often purchased separately, though some test kits come with temperature strips |
Included with the test |
|
Confirmatory Rate |
95% - 100% |
95% - 99% |
|
Cost |
$4 - $8 for single drug tests, $6 - $13 for multiple drug tests |
Both testing types are good options to use for the following:
- Pre-employment
- Workplace
- Random testing (ie. Employer)
- Court ordered
- Custody
- Military/Government
- Concerned parent/spouse/partner
- Probation/Parole
- Random employer drug screening
- Rehab
- Reasonable suspicion
- Post-accident
- Insurance
- Athletics
- Sobriety
Product Recommendations
Need help determining which test is right for you? We are happy to help!
GC/MS:
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