Clinical Laboratory - Chemical Pathology and Immunology
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Drugs of Abuse Testing
Drugs of Abuse Panels
Most "drugs of abuse" panels represent the most likely drugs to be encountered in persons coming to a hospital emergency department. Such drugs can include: amphetamines, benzodiazepines, cannabinoids, cocaine, ethanol, opiates, and phencyclidine.
Additional drugs of importance from acute toxicity include: acetaminophen, aspirin, ethanol, and methanol.
There are many substances that can potentially be taken by many different routes, but it is not possible to detect all substances with any one patient sample. It is not possible, even with sophisticated testing, to detect any possible compound in any sample without doing multiple tests and confirmatory tests. For example, running a sample of Coca-Cola® through the most sophisticated chemical testing will not reveal the secret formula.
False positive results may occur when there is cross-reactivity with other substances or similar substances. The methodologies for drug testing often detect groups of compounds, not specific drugs. For example, the test for opiates generally detects codeine, a constituent of many cough suppressants not considered a drug of abuse. "Amphetamines" could include ephedrine or pseudoephedrine found in prescription drugs or some herbal agents or over-the-counter drugs.
False negative results could be caused by timing in obtaining the specimen (too much time has passsed and the drug is metabolized and metabolites excreted), interfering substances, dilution, or substitution of another sample. Tampering is typically applied to a urine sample. Collection of a urine sample under observation is needed to avoid substitution, dilution, or adulteration. Adulterants are availble commercially, or common household items such as bleach may be employed. Urine dipstick analysis may reveal possible tampering: specific gravity below 1.003 suggesting dilution, or 1.000 suggesting water substitution, or pH out of the physiologic range (5.5 to 8.0) suggesting adulteration.
In general, most drugs are eliminated via renal excretion and appear in urine. Thus, a urine sample is often obtained to detect presence of a drug. However, the amount of urine produced over time is variable, and so drug concentration varies, so the actual amount of drug taken is difficult to assess in urine. Blood samples can be obtained when the amount of drug present must be determined. The blood concentration may vary according to metabolism and excretion, as well as the timing interval from drug administration.
For confirmation presence of a particular drug, additional tests may need to be run. Thus, the initial tests, including the drugs of abuse panel, is a screening test that needs to be interpreted in conjunction with patient presentation.