Why is it necessary is that instant drug testing results that screen non negative be sent to a lab for confirmation testing?

The primary reason being that all instant non-negative are only 97%-98% accurate which is shown on a package insert for any brand that is FDA Cleared.

All positive initial screens should be confirmed by GC/MS (Gas Chromatagraph/Mass Spectrometry) at a DHHS/SAMHSA certified laboratory.  This is due to the fact that any laboratory or on-site/instant drug screen, no matter how accurate, is not 100% accurate in identifying drugs of abuse in urine.  The initial screens react to multiple drug metabolites, and can under certain non-ideal situations, produce a positive result when the target drug is not present or is present at a level below the GC/MS confirmation cut-off level.  GC/MS confirmation is a more specific test that looks for and if present quantifies the target metabolite.  It is the GC/MS confirmation through a certified laboratory that will stand up in the court of law if the drug test result is challenged by the donor.

Why not just send instant drug testing specimens that are non-negative on initial screening to the lab to be screened again and then if non-negative it can go to GC/MS confirmation?

Sending the specimen to a lab as a “Blind screen” where the specimen will be screened by the laboratory, and if found positive by the lab screen, will be confirmed by GC/MS at the lab.  The error with this method is that the lab screen may produce a negative result on a specimen that is near the screen cut-off and that would confirm by GC/MS.  However, the specimen will never be sent to GC/MS for confirmation if the labscreen calls the specimen “negative”.  Therefore, any error or inaccuracy in the lab screen will result in a “true positive” specimen being ruled “negative”