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Methods of Drug Testing

    Drugs that have been introduced into the human body can be detected in urine, blood, other body fluids, and in hair. By far, though, the most common type of drug test is urinalysis (urine drug test). During urinalysis ( urine drug testing ), a person is asked to urinate into a specimen bottle. In most cases, a supervisor watches the collection of urine, either in person or in secret. The urine sample is then examined with chemical tests. The chemical test may be able to detect an illegal drug itself. More commonly, however, it detects a chemical produced during the breakdown of the drug in the body.

Any matter that enters the human body (includ­ing a drug) undergoes chemical changes. As an example, foods are broken down by the digestive sys­tem into simple sugars, amino acids, and other substances. These substances are then used by cells to manufacture energy and new compounds needed by the body, such as proteins. The substances formed during this breakdown process are known as metabolites. Most drug tests detect drug metabo­lites, not drugs themselves. The importance of this point for drug testing in the workplace is that a urine test typically shows what drugs were used at some time in the recent past, a few hours or a few days earlier. That drug use may have taken place during a person's private, nonworking time. It may tell little or nothing about the person's present ability to do his or her job.

    The human body takes different periods of time to break down and eliminate different substances. For example, amphetamines and their metabolites can be detected by drug tests for a period of about forty-eight hours after they were ingested, but n01 much longer. A person who has taken amphetamine_ at 8 P.M. on Monday will probably test positive fOJ the drug until Wednesday evening, but no later. Table 1 summarizes the periods during which various drugs can be detected. The data in this table are not absolute; studies sometimes show different figures for the lifetimes of various drugs.

A number of techniques are available for testing for drug metabolites. The test most commonly used today is the enzyme-multiplied immunoassay tech­nique (EMIT). In an EMIT test, a sample of urine is mixed with a chemical. The chemical combines with drug metabolites in the sample. The combination of chemical and drug metabolite produces a distinctive color. The presence of the color therefore shows that a drug metabolite is present. Another test that was once very popular and is easier to understand than EMIT is thin-layer chromatography (TLC). In TLC, a urine sample is mixed with other chemicals and passed down a thin strip of filter paper. Metabolites in the urine sample separate out as spots on the paper strip. The presence of an "amphetamine spot" on the paper indicates the presence of an ampheta­mine metabolite in the urine sample.

One problem with TLC is that it may produce incorrect conclusions. It may indicate the presence of a drug metabolite when that drug is not actually present (a false positive). Or it may indicate that no drug metabolite is present when it actually is in the sample (a false negative).

    False positives and false negatives may occur for a variety of reasons, such as the presence of chemi­cals in urine that act like drug metabolites, human error, or simply because chemical tests are not per­fect. For these reasons, any urine sample that produces a positive test for an illicit drug is tested a second time, using a different test. The confirmatory test is much more accurate than TLC or other pre­liminary tests. However, it also tends to cost more. The most common confirmatory test is known as a gas chromatography/mass spectrometry (GC/MS) test, which has a high degree of accuracy. A GC/MS test is somewhat similar to a TLC test. A urine sam­ple is mixed with a gas and passed through a detection chamber. Any drug metabolites in the sam­ple are separated out. The metabolites are then passed through a second device, a mass spectrome­ter, that confirms the presence of the metabolites. It shows which metabolites are present in the sample.

In the early days of drug testing, false results were common. In 1985, the Centers for Disease Control conducted a review of drug testing results from 1972 to 1981. It found false positive rates as high as 66 percent and false negative rates as high as 100 percent.5 The high rates of false positives and false negatives with TLC have led to the greater reliance on EMIT testing today.

    The rates of false positives and false negatives have decreased as drug testing procedures have improved. One study of thirty-one laboratories conducted by the American Association of Clinical Chemists, for example, found only 3 percent false negatives and no false positives.6 Critics of testing point out, however, that improvements in drug testing do not mean that the number of incorrect results have fallen to zero. That goal is probably not achievable in the real world. This problem can be especially severe during pre-employment testing. Many companies demand that applicants take a drug test during the application process. If they fail that test, they probably will not be hired. In such cases, no follow-up tests are conducted, and no false results are likely to be discovered.