A toxicology test (drug test or “tox screen”) looks for traces of drugs in your blood, urine, hair, sweat, or saliva. You may need to be tested because of a policy where you work or go to school. Your doctor could also order a toxicology test to help you get treatment for substance abuse or keep your recovery on track.
Urine tests are qualitative and detect the presence of multiple illicit drugs. Various drugs are detectable in the urine for varying amounts of time, and these results require careful interpretation and clinical correlation. Some substances or their metabolites may be present and result in true positives but are detected far beyond the time they would be clinically relevant. Urine screens may result in positives for a prolonged period after use or clinical effect.
- Cocaine: The test detects the metabolite benzoylecgonine quite accurately with little concern for false positives or negatives. However, the metabolite may be present up to 3 days after use while having most clinical effects within 6 to 12 hours of use.
- Amphetamines: Many false positives to amphetamines exist, including antihistamines, decongestants, antidepressants, or acid-blockers, and may be detected 1 to 3 days after use.
- Marijuana: Common over-the-counter analgesics like ibuprofen and naproxen can cross-react with this assay, creating false positives.
- Phencyclidine (PCP): False positives exist, notably with ibuprofen, dextromethorphan, and tramadol, and the time for detection can last 1 to 2 weeks after use.
- Opioids: Many opioids can be missed on routine screening, generating significant false negatives, and there are reports of false-positive results in the presence of poppy seeds and quinolone antimicrobials. Opioids are detectable for 1 to 4 days after use.
- Benzodiazepines: These results are likely among the least clinically useful screens. A few false positives exist, but false negatives are far more common, as most assays detect the specific metabolite oxazepam, while multiple benzodiazepines with unique metabolites are likely to be missed. Diazepam is metabolized to oxazepam and would be identified but might be present for up to 4 weeks after use. On the other hand, midazolam, lorazepam, and alprazolam all metabolize into other substances and therefore, would remain undetected.
Serum testing may be more open to the interpretation of timing due to quantitative testing, but it can be unclear if concentrations are rising or falling, and pharmacokinetics and metabolism of substances can be very difficult to predict, necessitating repeat sampling.