Physicians must understand the process of any kind of urine drug screening that will have the implication of discharging a patient or altering their treatment plan. Urine drug screening was designed to detect illicit and/or licit non-prescribed drug abuse, not to monitor adherence to treatment regimens.
Physicians must be sure that they are getting a high-quality laboratory process. The Department of Health and Human Services (DHHS) Substance Abuse and Mental Health Services Administration (SAMSHA) has a certification program for analytical laboratories . It is important to understand how the specimen is collected, what is being prescribed, and the retention times. Many prescribed opioids are not part of a standard UDS; for example, you must specifically ask for oxycodone and hydrocodone. With a formalized quality-controlled collection procedure, UDS can be a valuable tool - for example, an unexpected result can support decision to refer to a specialist. Interpretation requires information - physicians should get to know their laboratory director and consider a medical review officer consultation.
Urine drug screening is typically a two-step procedure. The first step uses proprietary immunoassays to detect the presence of a drug or metabolite in the urine. Specimens identified as positive on the initial immunoassay test are confirmed using gas chromatography/mass spectrometry . There can be multiple reasons for a negative urine drug screen, including the cutoff points used by the testing laboratory. The specificity and sensitivity of immunoassays vary depending on the assay type and specific test performed. Antibodies are seldom specific to a single drug or drug metabolite; therefore cross-reactions cause false positive results. Positive results based on immunoassay tests alone are referred to as "presumptive positives" and must be confirmed using a different assay technique. Be aware that a positive opiates screen cannot distinguish between morphine, codeine, or heroin . Even confirmation by gas chromatography/mass spectrometry (GC/MS) has variables in the assay procedure that affect its sensitivity, specificity, and reliability . Although GC/MS can quantitate the level of drug in the urine, no inference can be made about how much was taken because the levels of drugs or drug metabolites in urine are affected by numerous factors.
Summary of reasons for false negative/positive UDS
2. "Presumed, false positive" results may be due to the tremendous variability in specificity and sensitivity of assay type and the specific test performed a. Assay methods being used b. Laboratories being involved c. Technicians being a human source of error
N.B. Urine drug levels are not related to blood levels either detected or not detected
An example of a common error of interpretation is when the UDS of a patient taking a hydrocodone preparation is reported as "positive" for opiates, which may occur because there is some cross reactivity. The GC/MS, however, confirms the presence of hydrocodone and hydromorphone. Therefore, the patient is accused of abusing street drugs or doctor shopping. The solution is to understand the metabolism of hydrocodone and, therefore, know that a fraction of hydrocodone is metabolized to hydromorphone. Also, in order to eliminate the identification of most individuals who have ingested poppy seeds, the DHHS raised the testing levels for opiates from 300 to 2,000 ng/mL. They also established a new requirement to test for 6-MAM, a metabolite that comes only from heroin, for specimens that test positive for morphine .
Cases have been documented of cocaine ingestion by drinking tea made from coca leaves. Although such tea may be available for purchase by (apparently) unknowing consumers, the product-containing active cocaine and/or related metabolites-is illegal under US DEA and FDA regulations, and does not constitute an "alternative valid medical explanation" .
Cocaine is a local, vasoconstrictive anesthetic, not resembling other local anesthetics that are more commonly employed in ENT procedures. A positive urine test through medical use can be checked through documented medical records, prescription forms, or contact with the prescribing practitioner . Immunoassay screening tests cross-react with various amphetamine-related drugs that are not abused, for example, dopamine, isoxsuprine (a vasodilator), and ephedrine (an asthma medication). GC/MS confirmation distinguishes the cross-reacting compounds, assuring that results are not false positive. The Vicks® Vapor Inhaler contains desoxyephedrine, which is the l-form of methamphetamine. A separation of d- and l-methamphetamine isomers should reveal 100% l-methamphetamine following Vicks® Vapor Inhaler use; more than 20% of d-methamphetamine suggests a source of d-methamphetamine other than the inhaler .
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