How does the drug testing work for a urine test?

Urine Drug Screening: Urine can contain drug byproducts or even the drugs themselves. Urine drug screening tests are designed to measure all kinds of drugs that are abused and can tell the testing agency if you have consumed the drugs in question. For example if you are not to be on opiates then the urine screening test would be positive for opiates if you have had them in the recent week or so.

Check the link: These toxicology screen tests usually test for a range of substances or substrates from drugs like morphine, cocaine, marijuana etc. The tests are sensitive enough to detect the molecules if there is a certain concentration of the drug or it's substrate in the urine. If positive, the confirmatory test is done to quantify the substance. Here is more info: http://www.Aafp.Org/afp/2010/0301/p635.Html.