Immunoassay techniques to test for the presence of specific drugs in the body rely on immunological principles. When various drugs enter the body, the immune system develops a protective antibody. Immunoassay techniques measure the presence of antibodies in the urine associated with particular drugs. There are several varieties of immunoassay including radioimmunoassay, enzyme multiplied or multiple immunoassay, and free radical assay. Although immunoassay evidence will likely be admitted even over an objection based on the Frye doctrine, such evidence is still vulnerable to "weight" attacks. Weight attacks are available whenever a prosecutor offers to prove that there was an illicit substance in the defendant's urine, the defendant consumed an illegal substance, or the defendant was impaired at a particular time. Immunoassay tests are nonspecific. They will yield positive test results with other substances because they are "cross-reactive" with other substances. False positives are so common that an attorney has much ammunition with which to persuade the trier of fact that a positive test does not establish the presence of an illegal substance in the defendant's urine. Further, unless the laboratory uses a realistically high cutoff value, the attorney may succeed in convincing the trier of fact that it is unwarranted to infer that the defendant consumed an illegal drug. It is certainly feasible for the attorney to persuade the trier of fact that the test by itself cannot prove that the defendant was impaired at a particular time. 47 footnotes