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Clinical drug testing analyzes plasma, serum, or urine to detect the presence or absence of a drug or its metabolites. As the metabolization rate of drugs differs, the detection window for specific drugs or metabolites varies. Clinical drug testing plays an essential role in managing poisoning because the self-report of the drugs taken is often unreliable. The same is true in treating addiction disorders because clinical examination, patient self-reporting, and hetero-anamnesis will underreport the actual incidence of substance use. Drug testing can be indicated in cases of suspected overdose or when monitoring abstinence in patients treated for addiction or in pain management clinics. No universal standard currently exists in clinical drug testing for addiction identification, diagnosis, treatment, medication monitoring, or recovery. Guidelines do exist for laboratory analyses of poisoned patients.[1][2] In poisoning cases, the indications for laboratory assays are to confirm the suspicion of poisoning when this is in doubt and to influence patient management. Indications for laboratory assays may include establishing or eliminating the need for further investigations or administration of antidotes, hemodialysis, or other invasive extracorporeal epuration methods. Clinical drug testing may also be needed to determine if the cessation of treatment is indicated or to plan the re-institution of chronic therapy. In the intensive care unit, clinical drug testing is used to aid in the diagnosis of brain death and to determine the suitability of potential organ donors. The use of laboratory investigations out-of-hours should be restricted to those instances when an urgent result is needed to guide immediate patient management. It may also be appropriate to obtain and store samples for later analysis.[1]

When a drug is taken orally, it is absorbed from the gastrointestinal tract, distributed to the rest of the body, metabolized in the liver and other organs, and eliminated, mainly in the urine. These processes occur at different speeds for different drugs. Therefore, for the target analyte, be it the parent drug or metabolite, the first and subsequent detection times will differ for the various drugs. For example, for some drugs like amphetamines, the parent drug will be detected in urine; metabolites are detectable longer than the parent drug for most other drugs. The primary target metabolites are 11-Nor-9-carboxy-delta-tetrahydrocannabinol (THCCOOH) for cannabis, benzoylecgonine for cocaine, and morphine for heroin. Drugs generally first appear in the urine one to two hours after intake. After a small drug dose, a drug can be detected in the urine for one to three days; after heavy, chronic use, amphetamines can be detectable for ten days, cannabinoids for three months, cocaine metabolites for three weeks, and heroin metabolites for 11 days.[10] In plasma, the detection times are shorter, often one to two days.

Interpretation of drug tests is notoriously difficult because of the limited sensitivity and specificity of the assays and the variability among assays. Results of urine drug tests should be discussed with each patient, and decision-making surrounding urine drug test values should include a multidisciplinary team and the patient. Because of the complex nature of result interpretation and test ordering, a close working relationship must be established with the laboratory. Clinicians should be encouraged to discuss these issues with a toxicologist or clinical chemist for a correct interpretation and to select further tests. A recent study found that 25 of 160 interpretations by clinical providers differed from those of the laboratory toxicologist.[27] With national policies directing the limitation of opioid medication use, surgeons strategize pain control and surgery protocols to limit opioid medication abuse.[28] Pain management clinicians also utilize clinical drug testing in chronic pain patients to ensure conformance to opioid use contracts and avoidance of controlled substances.[29]