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: 




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%


$4 - $8 for single drug tests,

$6 - $13 for multiple drug tests 

$69 - $89+


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! 




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