Interpretation

In an ideal world, testing biological samples should lead to definitive answers. However, test results sometimes lead to more questions than answers. Adept interpretation of results will lead to improved clinical care or to more surety in consultative cases. Interpretation depends on awareness of which test was used and its meaning to the situation.

In most settings, the primary purpose of drug testing is to identify individuals who are using illegal or illicit drugs. Falsely accusing someone of using drugs is highly problematic and undermines the testing program. Similarly, not being able to identify active drug users because of false-negative results renders a program of limited value. It does not deter use or identify users. This is so both for the emergency room physician wondering if the agitated patient used PCP, and for the consultant to the local college track team. For these situations, highly sensitive qualitative screening tests should be employed, even if this leads to some false-positive results. On the other hand, definitive tests should have the highest level of specificity: They should exclude as many true negatives as possible. For nonusers who are subjected to drug testing, issues related to false-positive results are of great concern. Questions addressing which foods, prescribed medications, dietary supplements, or potentially secondhand marijuana smoke could result in a positive test are common, and some laboratories have responded by raising the level required in order to render a positive test result.

With the proliferation of private laboratories and commercially manufactured kits, there has grown to be some interlaboratory variation in standards and thresholds for results. The industry even has its own trade organization, the Drug and Alcohol Testing Industry Association (DATIA). The major concern in drug testing occurs with the reporting of laboratory results. Unlike National Institute on Drug Abuse (NIDA)-certified testing of the Standard Drug Panel (see Table 4.2), clinical drug testing for drugs of abuse currently has no standard technical criteria, no standard screening cutoffs for positive tests, no confirmation cutoffs, no chain-of-custody requirements, no blind proficiency submission requirements, and no certification programs. As a result, a sample testing posi-

TABLE 4.2. "NIDA 5" or the DOT Standard Drug Panel 1990

Drug

Cutoffs (ng/ml)

Cocaine Cannabinoids PCP Opiates

Amphetamines

300/150

50/15

25/25

2,000/2,000

1,000/500

tive in laboratory A may be reported as negative by laboratory B, based on different cutoff levels. This is not a new development (Hansen, Caudill, & Boone, 1985). Unfortunately, little progress has been made in correcting it over the past 17 years. The issue is not the type of test administered, or poor-quality laboratories, but rather the nonstandardized threshold for reporting a test as positive.

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