from: Maltby LL Drug Testing - A Bad Investment (1999), pg. 18. http://www.doctordeluca.com/Library/PublicHealth/DT-BadInvestment-ACLU99.pdf  (accessed: July 29, 2002).

THE PROBLEM OF “FALSE-POSITIVES”
Rather than searching for drugs, urine tests search for drug metabolites – inactive drug by-products that the body produces as it processes drugs for excretion. Typically, urine samples are first screened by immunoassay, a technology that recognizes drug metabolites based on a “rough outline” of their chemical structure. The benefit of immunoassay screening is that it is quick and fairly inexpensive. The problem is that it may not distinguish between drug metabolites that have closely similar structures. Consumers of over-the-counter decongestants, for example, may have positive tests for amphetamine. Antihistamines can produce false-positives for methadone. Because of this, a positive result on an immunoassay screen does not mean necessarily that a person used an illegal drug. If a drug testing program relies on immunoassay screening alone, it will always identify some people as illegal drug users when they are not.

A second (more costly) test, gas chromatography/mass spectrometry (GC/MS), gives more precise readings. It can distinguish cold remedies from amphetamine and antihistamines from methadone. This second test, however, does not eliminate the possibility of false accusations. Codeine, for example, produces the exact same drug metabolite as heroin. Marinol produces the same drug metabolite as THC, an active ingredient of marijuana. Rather than false-positives, these are
“innocent positives.” The drug metabolite being searched for was present, but it appeared in the urine following use of a legal medication. 

There are even some foods and nutritional supplements that produce the same metabolites as illegal drugs. After consuming poppy seeds, for example, people have morphine metabolites in their urine. After consuming hemp oil products, people have THC metabolites in their urine. Neither of these products produces a drug effect, regardless of how much is consumed. But because they contain very low concentrations of the drugs that are present in heroin and marijuana, they can result in a positive drug test. 

Through technological advances, drug testers have solved many of the false-positive problems that plagued early programs. But they haven’t solved them all, and additional problems will surely emerge as new medications and nutritional supplements are available. Moreover, at every stage of the process, from urine collection to final reporting, the potential for “human error” will always exist. In short, no drug testing program will ever be able to completely avoid making false accusations of drug use. 


Hair Testing Isn’t Better... It’s Worse 
All of the problems with urine testing are present with hair testing as well. Plus, hair testing has additional problems of its own. For one thing, small amounts of drugs can be more easily detected in some types of hair than others. All else being equal, dark-haired people are more likely to test positive than blondes, and African Americans are more likely to test positive than Caucasians. It is still not known whether test results are affected by hair bleaching or the use of various hair products. In addition, there is no way to adequately control for the possibility of external contamination. “Passive exposure” to drugs in the environment, particularly drugs that are smoked, may lead to “innocent positive” results on hair tests.

SOURCES: FORENSIC DRUG ABUSE ADVISOR8, 10 (NOV/DEC
1996); FORENSIC DRUG ABUSE ADVISOR9, 4 (APRIL 1997).



Every Worker a Scientist?
Some employers, including the New York City Police Department, have issued an order prohibiting workers from consuming any food product or nutritional supplement, or using any cosmetic, that might trip a positive result on a drug test.

A police union official complained, “To expect police officers to read the list of ingredients on a bag of snack food, many of which are named in chemical terms, is to ask the impossible.... We are not scientists or doctors. We are police officers.”

SOURCE: MICHAEL COOPER, “NEW POLICY TAKES ON HEMP
OIL,” NEW YORK TIMES, 22 JULY 1999, P. B3.