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Urine drug screens are frequently ordered on patients who exhibit symptoms of intoxication, experience trauma or have a history of drug ingestion. Most hospital laboratories use immunoassays to detect drugs because they are relatively simple to perform, have high sensitivity for drugs of abuse and provide rapid turnaround time. One example is the Triage® TOX Drug Screen, which is a fluorescence immunoassay for the qualitative detection of 10 distinct drug classes in urine, including acetaminophen/paracetamol, amphetamines, methamphetamines, barbiturates, benzodiazepines, cocaine, methadone, opiates, phencyclidine, tetrahydrocannibinol (THC) and tricyclic antidepressants (TCA). Screening assays for drug of abuse are designed to detect urine drug levels above a predetermined cutoff concentration. The threshold concentration, above which each drug class will be detected by the Triage® TOX Drug Screen, is summarized in the following table. The acetaminophen/paracetamol assay will yield positive results when acetaminophen/paracetamol is ingested at or above therapeutic doses.
Reference value is a negative result for all drugs tested. A positive result on a drug screen is not evidence that the person providing the specimen was under the influence of that drug at the time the specimen was submitted. A positive result only indicates previous use of the particular drug that was detected in the urine. Drug Classes Amphetamines: Amphetamines are abused because they are psychomotor stimulants. They increase wakefulness, alertness and energy, suppress appetite and produce an overall euphoria. Amphetamine and methamphetamine can be detected up to 48 hours after single use and 7 to 10 days after chronic use. Most amphetamine immunoassays are designed to detect amphetamine and methamphetamine, while some are designed to detect MDMA and MDA. Drugs commonly considered to be illicit amphetamines include the D-isomers of amphetamine, methamphetamine, phentermine and methylenedioxymethamphetamine (MDMA, Ectasy) and its metabolite methylenedioxyamphetamine. Many over the counter sympathomimetic amines present in cold medications such as ephedrine, pseudoephedrine, phenylpropanolamine and phenylephrine may produce false positive screening results. Herbal supplements containing ephedra may produce a positive amphetamine reaction. Barbiturates: These drugs are powerful central nervous system depressants. Low doses induce relaxation, moderate doses induce sleep and high doses induce anesthesia. Tolerance of some of the barbiturates' desired effects develops rapidly. Physical dependence develops when a user's daily intake exceeds a threshold value; once a user is physically dependent, abrupt withdrawal can precipitate symptoms severe enough to cause death. Short acting barbiturates can be detected up to two days after use. Phenobarbital can be detected one to three weeks after use. Cocaine: is a potent psychomotor stimulant that results in a state of increased alertness and euphoria. These effects are attributed by the blocking of dopamine reuptake by nerve synapses. Sympathomimetic effects include increased blood pressure, heart rate and body temperature. The route of administration has a marked impact on its addictive potential. The more rapid the rate of increase in drug levels in blood and brain, the greater the risk of addiction. Intravenous and smoked cocaine are much more addictive than intranasal snorting. Cocaine is rapidly metabolized in the blood and liver after inhalation and only small quantities of parent drug are excreted in urine. Benzoylecgonine and ecgonine methyl ester are the two main cocaine metabolites; neither is pharmacologically active. The half life of cocaine ranges from 0.5 to 1.5 hours, ecgonine methyester from 3 to 4 hours and benzoylecgonine from 4 to 7 hours. Because of its longer half life, most screening immunoassays have been designed to detect benzoylecgonine. Opiate: Opiates are powerful analgesic and sedative drugs that affect the cardiovascular and gastrointestinal systems. Opiates have a high abuse potential because they produce euphoria, relaxation and feelings of well being. Long term use can lead to tolerance and both physical and psychological dependence. Most immunoassays detect morphine with varying levels of crossreactivity to codeine, hydrocodone, and hydromorphone. Heroin is metabolized to morphine and can be detected for two days after single use and four to five days after chronic use. Oxycodone, oxymorphone, meperidine and methadone cannot be reliably detected by the majority of screening assays. Ingestion of poppy seeds may produce a positive opiate reaction. Benzodiazepines: These drugs are widely prescribed as anxiolytics, sedative-hynotics, anticonvulsants and for treatment of obsessive compulsive disorder. They may cause cognitive impairment. Chronic benzodiazepine use increases the risk of developing dependence and abuse. Most screening immunoassays detect all benzodiazepine drugs that are metabolized in the body to oxazepam, temazepam, or nordiazepam. However, the large number of different chemical groups that may be attached to the benzodiazepine nucleus make it difficult for screening assays to detect all drugs in this class. Some drugs such as midazolam, chlordiazepoxide and flunitrazepam may not be detected by many manufacturers' assays. Screening assays cannot distinguish between individual benzodiazepines. Benzodiazepines are undetectable in the urine after single use, but some can be detected for one to five weeks after chronic use. Over the counter remedies can produce false positive results in some benzodiazepine screening assays. Phencyclidine: PCP is a dangerous drug of abuse that produces visual and auditory hallucinations, feelings of dissociation, intense euphoria, and distortions in perception of time, space and body image. It is taken orally, inhaled or injected. PCP can be detected one week after single use and two weeks after chronic use. Dextromethorphan and Doxylamine (Unisom) have been reported to cause false positive results with PCP screening immunoassays. Over the counter remedies can produce false positive results in some phencyclidine screening assays. Cannabinoids: Marijuana is a mixture of dried leaves and the flowering tops of the plant Cannabis sativa and is the most widely used illicit drug. Delta-9-tetrahydrocannabinol (9-THC) is the principal psychoactive ingredient in marijuana. Street preparations of marijuana commonly contain 0-5% 9-THC. Hashish, the resinous coating from Cannabis sativa L. leaves and flowers, contains 5-15% 9-THC. The compound is quickly and effectively absorbed by inhalation or from the gastrointestinal tract and is almost completely metabolized. Fatty tissue absorbs 9-THC and then slowly releases it into the plasma. Most immunoassays can detect 9-THC two to seven days after single use and one to two months after chronic use. False positive reactions for THC occur most commonly because several prescription medications including Clozaril, Propulsid, Protonix, Paxil, Tegretol and Zocor cross-react with the anti-THC monoclonal antibody used in some screening assays. Confirmation of Positive Drug Screens The major problem with all rapid immunoassays is their less than perfect specificity for each drug class. Prescription and over the counter medications, as well as herbal supplements, may cause false positive results. Physicians need to be aware of the limitations of urine drug screens. If a falsely positive drug screen is suspected, a confirmatory drug screen should be ordered. As a reminder, many laboratories attach a comment to urine drug screen results such as, "This drug screen provides presumptive results for medical purposes only. False positive results may occur. Physicians should order confirmatory testing on this sample if the results are considered clinically significant". Sample Collection Freshly voided urine specimens should be collected in a clean, previously unused glass or plastic container. If the specimen will not be tested immediately, it should be refrigerated at 2 °C to 8 °C for a maximum of two days. Stat drug screens are available 24 hours a day. Turnaround time is approximately 30 minutes. Urine specimens are stored for at least one week, so that confirmatory testing can be done at a later time. Specific plasma drug levels are also available if overdose of a particular medication is suspected. |
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