Like all diagnostic tests, it should not be assumed that a negative drug screen indicates with perfect accuracy that no illicit drugs have been used by the patient. Although rapid urine drug tests allow the quick detection of a wide variety of drugs, there are other factors to consider when interpreting the results of these tests. General urine drug tests should only be used as an initial screening mechanism for determining whether a drug of abuse is present in a patient. More-specific drug tests should be considered after an initial screen is performed and analyzed. A urine drug screen is most valuable when the diagnosis is unknown, that is, when one is not certain whether symptoms are being caused by drugs or by disease.

There are several techniques used for drug detection, including chromatographic methods, immunoassays, and chemical and spectrometric techniques. Chromatography is used for broad-spectrum analysis, whereas immunoassays are used for specific analysis. Being aware of what drug coverage is available in particular toxicology screens is helpful. Most tests identify analgesics, amphetamines, antidepressants, barbiturates, cocaine, ethanol, and opiates. Drugs that are not commonly found in standard drug screens include bromide, carbon monoxide, chloral hydrate, clonidine, cyanide, organophosphates, tetrahydrozoline, colchicines, cyanide, iron, β-blockers, calcium-channel blockers, clonidine, and digitalis. Note that traces of many drugs persist for lengths of time that may not be clinically relevant depending on the situation. Realizing that false-positive and false-negative results are not uncommon is essential for making clinically sound decisions. As such, confirmation analyses may be required. In addition, inquiring about the patient’s legal drug use can help to clarify toxicology screening results. Aspirin and acetaminophen, for example, are very common analgesic ingredients in many medications, so measuring their prevalence should be considered (Table 76.1).