Presumptive screening

This is the first step in the drug testing process. The reason the test is presumptive, is that it indicates the likely presence of drug. It may not target a drug specifically and absolutely to the exclusion of all others. At STL, the presumptive screening is performed on an automated chemistry analyzer, interfaced with the laboratory computer system. Presumptive screening is rapid and fairly inexpensive, but its purpose is to eliminate the negative specimens and narrow down the scope of more costly confirmatory testing.