How is toxicology screening performed in the evaluation of cocaine toxicity?

Updated: Sep 01, 2018
  • Author: Lynn Barkley Burnett, MD, EdD, LLB(c); Chief Editor: Sage W Wiener, MD  more...
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Answer

On a urine drug screen, drain cleaner or bleach added to urine can mask cocaine; alkaline urine may raise this suspicion. Desipramine and amantadine, prescribed to reduce cravings for cocaine, may cause false-positive results on urine tests for amphetamines. No substances result in a false-positive urine drug screen for cocaine, though false positives may still occur from switched or mislabeled specimens.

Urine, blood, gastric contents, and unknown substances found on patients, such as on a mustache, may be sent for toxicologic evaluation. Include the patient's clinical history and differential diagnosis of the toxins in question to guide the laboratory evaluations. High plasma cocaine concentrations are rarely observed because cocaine has a short half-life of 30-45 minutes. Furthermore, numerous studies have demonstrated that toxicology screening rarely changes the clinical treatment of patients. Although concentrations higher than 1 mg/L are generally associated with toxicity, deaths have been reported with blood levels of 0.1-20.9 mg/L. Because of this wide range of toxicity, quantitative blood levels of cocaine or metabolites are generally not clinically useful.

Cocaine exhibits first-order kinetics over a wide dose range; therefore, after 5 half-lives (approximately 4 h), virtually all of the cocaine taken should have been converted to its metabolites. Hollander et al concur, indicating that urinary cocaine may be detected for 4-8 hours after a single intranasal dose. However, Lewin, Goldfrank, and Weisman maintain that most cocaine is excreted in the urine within 24 hours of ingestion. [38]

Benzoylecgonine, which may induce neurotoxicity, ischemia, and arrhythmias, [39] may be present in urine for as long as 60 hours after single use and for as long as 22 days after the cessation of heavy cocaine use. If the ratio of benzoylecgonine to cocaine found in the urine is less than 100:1, either the cocaine was ingested less than 10 hours before collection of the sample or ongoing liberation of cocaine is occurring from a body package. The “cocaine” assay on most clinical urine drug screens are for benzoylecgonine and not cocaine itself.


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