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What makes a drug test legally defensible?

Any drug test performed with any specimen (urine, hair, etc) should be tested with an initial screening method and then followed up with a confirmation test if the screening result is non negative. 

The lab performs initial drug screening 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 non-negative or negative result, indicating the presence or absence of detectable drug. 

The results of the screening test if non-negative must be confirmed with a separate more specific testing methodology.  GC/MS is the abbreviation for gas chromatography/mass spectrometry, the confirmation testing methodology labs use to confirm non-negative drug screen specimens as Positive or Negative. 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. The specific drug or metabolite of the drug is measured with a quantitative value. 

The Positive lab results should always be review and verified by a Medical Review Officer (MRO) to rule out any legally prescribed medications which may have the positive result.  A Medical Review Officer (MRO) is a person who is a licensed physician and who is responsible for receiving and reviewing laboratory results generated by an employer's drug testing program and evaluating medical explanations for drug test results.  An MRO should always be used in the drug testing process, this avoids any question of a false positive.