A positive drug test for cocaine, often conducted via urine or saliva screening, requires careful interpretation. Initial tests are designed to be highly sensitive to avoid missing potential drug use. This sensitivity, however, can sometimes lead to a presumptive positive result due to factors other than illicit cocaine use. Understanding how other substances can interfere with the results is important. This article explores the non-cocaine-related factors that can cause a positive result on an initial drug screen.
The Target Metabolite: Benzoylecgonine
Cocaine testing relies on detecting a breakdown product called Benzoylecgonine (BE) rather than the cocaine molecule itself. Cocaine has a very short half-life of about 1.5 hours, meaning it is rapidly metabolized and quickly eliminated from the bloodstream. In contrast, BE has a much longer half-life, ranging from 5.5 to 7.5 hours, making it detectable in urine for two to four days after use.
BE is the primary marker targeted in most drug testing panels due to this difference in elimination time. Initial screening tests, typically immunoassays, are calibrated to detect BE at a specific concentration threshold, known as the “cut-off level.” Standard screening cut-offs for BE in urine are often set at 150 nanograms per milliliter (ng/mL) or 300 ng/mL, depending on the testing program.
A positive screen indicates that the concentration of substances reacting with the test antibodies is above this pre-determined cut-off level. The high sensitivity of these antibody-based tests, which are designed for rapid initial results, makes them susceptible to cross-reactivity from other compounds.
Cross-Reactivity from Prescribed Medications
A common reason for a presumptive positive screen is cross-reactivity, where test antibodies mistake a structurally similar legal substance for the target metabolite, Benzoylecgonine. This interference is almost exclusively seen in the initial immunoassay screen because some pharmaceutical compounds share chemical features with the cocaine metabolite.
Certain classes of prescribed medications have been reported to cause these false screens. Local anesthetics, such as lidocaine and procaine, are frequently used in dental work or minor procedures. These possess a chemical structure that can resemble the cocaine molecule. While research suggests lidocaine may not cause a false positive on standard immunoassay screens, the principle of structural similarity leading to interference remains a possibility for various compounds.
Antidepressants and other psychotropic drugs also represent a category of concern. Specific examples include certain selective serotonin reuptake inhibitors (SSRIs), which have been associated with false positives for various drugs. The antipsychotic quetiapine (Seroquel) has also been noted to potentially interfere with immunoassay drug testing. These are false screens that are typically resolved during the subsequent confirmation process.
Environmental and Ingestion Triggers
Substances ingested or encountered in the environment can also lead to a positive test result. The most widely cited example of ingestion is the consumption of coca leaf tea, often called “mate de coca.” This herbal infusion, popular in certain South American countries, is made from the leaves of the Erythroxylum coca plant, the natural source of cocaine.
Ingesting coca tea introduces trace amounts of cocaine and its metabolites directly into the body. Studies show that consuming this tea can result in urinary BE concentrations that exceed the cut-off level, sometimes reaching over 4,000 ng/mL, leading to a positive result. This is a direct biological positive, not a false positive, because the BE is genuine, even though the user did not use illicit cocaine.
Environmental contamination is another potential source for low-level detection, particularly in hair testing. Cocaine is a highly static substance that transfers easily from the environment onto surfaces and hair. Contamination can occur through passive exposure, such as being in a highly contaminated environment or handling contaminated objects. While passive exposure is generally unlikely to cause a positive urine test given standard cut-off levels, it is a significant consideration in hair testing and can result in trace amounts of BE being detected.
The Confirmation Process
A positive result on an initial immunoassay screen is considered presumptive and is not a definitive finding of cocaine use. Any specimen that screens positive must undergo a second, more sophisticated test to confirm the presence and concentration of Benzoylecgonine. This confirmatory analysis is performed using a technique known as Gas Chromatography/Mass Spectrometry (GC/MS) or Liquid Chromatography/Mass Spectrometry (LC/MS).
The confirmation process is considered the standard because it functions differently than the initial screen. Chromatography (GC or LC) first separates all the individual chemical compounds in the sample. Then, the Mass Spectrometry (MS) component identifies the separated substances by creating a unique molecular fingerprint for each compound.
This two-step process eliminates the possibility of a false positive caused by cross-reactivity. The GC/MS or LC/MS method can specifically distinguish the precise molecular structure of Benzoylecgonine from any structurally similar compounds, such as prescribed medications, that may have caused the initial screen to flag positive. The result is considered legally defensible and conclusive only after the BE is identified at or above the confirmatory cut-off level, which is commonly set at 100 ng/mL.