While screening tests are very accurate and reliable tests, they are not 100% accurate. Samples which screen positive are presumed to be actually positive, but the specific drug must be confirmed by a more specific method. The method employed by the Toxicology Section for all confirmations is gas chromatography/mass spectrometry (GC/MS).
In confirmatory tests, a procedure specific to the class or family of drug for which the sample screened positive is employed to "wash" or separate the drug itself from the matrix, i.e. blood or urine, in which it resides. The procedure of separating the drug from the sample is called an extraction. The Toxicology Section has different procedures for each class of drug. The general scheme of an extraction is to adjust the pH of the sample, expose the sample to an organic solvent to wash out the drug, collect and concentrate the organic solvent, and derivatize the drug in the sample. Some extractions have a but a few steps in the scheme, while others have many steps. The extraction process is one of the most time-consuming aspects of drug testing.