In general, amphetamine immunoassays produce frequent false-positive results, whereas cocaine and cannabinoid assays are more specific.1,18 Common over-the-counter medications, including nonsteroidal anti-inflammatory drugs, decongestants, and antacids, can yield false-positive results, highlighting the need to obtain a comprehensive medication list from patients, including over-the-counter and herbal medications, before ordering UDS. Because of the complexity of cross-reactivity, it might not be possible to identify the source of a false-positive result.14
Patient variables. Intentional effort to skew results is another source of interference. The frequency of this effort varies by setting and the potential consequences of results—eg, employment testing or substance use treatment—and a range of attempts have been reported in the literature.21,22 Common practices are dilution, adulteration, and substitution.20,23
- Dilution lowers the concentration of the drug of interest below the detection limit of the assay by directly adding water to the urine specimen, drinking copious amounts of fluid, taking a diuretic, or a combination of these practices.
- Adulteration involves adding a substance to urine that interferes with the testing mechanism: for example, bleach, household cleaners, eye drops, and even commercially available products expressly marketed to interfere with UDS.24
- Substitution involves providing urine or a urine-like substance for testing that did not originate from the patient.
Methods to minimize patient-related interference include observed collection and specimen validity testing for pH, creatinine, and adulterants (TABLE 2).1,15 Efforts to detect patient interference must be balanced against concerns about privacy, personnel resources, and the cost of expanded testing.14,19,20
Additional aspects inherent to the testing process, such as cutoff concentrations and detection windows, can lead to interference. Laboratories must set reporting cutoffs, and specimens with a drug concentration present but below the cutoff value are reported as a negative result. Detection windows are complex and are influenced by inherent properties of the drug, including metabolic pathway and route and frequency of use.1 A given patient might well be using a substance, but if the specimen was obtained outside the detection window, a false-negative result might be reported (TABLE 31,23).
Managing test results
Appropriate management of UDS results is built on the foundation of understanding the testing mechanism, selecting the correct test, and properly interpreting results. Drug testing is, ultimately, a therapeutic tool used to monitor treatment, provide reinforcement, and explore substance use behavior; results of testing should be employed to achieve those objectives.1,4,14 A negative or expected UDS result can be utilized as positive reinforcement for a patient who is adherent to the treatment plan—much the way objective weight loss in an obese patient can provide encouragement to continue lifestyle changes.
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