They have different tests like 8 panel or a 12 panel. They have to test specifically for a certain drug and these panels do.
Drug tests work by looking for metabolites IE marijuana gives off something like 31 specific metabolites in your urine when you metabolize the drug. If those are present it will signal a postive result.
Its either in your system or not they cant really tell how much or long you did it last and most drugs are out of your system within 4 days.
There are other tests like hair follicle that can find metabolites from up to 6 months back.
However If your urine test is postive they can send it off to get GC analysis from there they can tell how much of the drug is still in your system.
Last edited: Feb 10, 2020