According to the NIDA practices, test samples are screened for drugs first. The screens are designed with the NIDA cutoffs. This means that a sample containing drugs below the NIDA cutoff will be reported negative on the screen. Under NIDA practices, the results of a screen is only presumptive and must be confirmed. Only the samples that test positive for a drug on the screen are sent to the lab for confirmation.
Immunoassays use antibodies to detect the presence of drugs.
An antibody is a protein that will react only when combined with a specific substance or very similar substances. In using immunoassays in drug testing, the targeted drug acts as the antigen. Immunoassays were developed in the 1960s for drug testing, pregnancy testing and for testing for various medical conditions. The immunoassay is highly accurate at detecting very small amounts of antigens. While there are several types of immunoassay, they all rely on similar techniques of chemistry, involving the reaction of antigens and antibodies.