This is one of the most common questions that we are asked in the lab. Most drugs are metabolized by the liver to more water soluble forms (metabolites) in order to be excreted in the urine by the kidneys. Many classes of drugs (e.g. opioids, stimulants) are excreted completely in urine in 2-3 days. Others (e.g. Benzodiazepines) have a longer half-life in blood and are converted into metabolites that also have a long half-life. Benzodiazepines can be detected up to a couple weeks in urine. Still others (e.g THC) are more fat-soluble and can be slowly released over time. For a casual user (once a week), the THC metabolite is detectable for approximately 3 to 4 days. In a chronic user (use every day), excretion of this metabolite may take several weeks. Little is known about the detection window for the newer synthetic cannabinoids (aka spice) designer stimulants (aka bath salts), but the detection window is likely comparable to the similar types of drugs such as THC (for synthetic cannabinoids) or amphetamine (for designer stimulants). The attached table may be helpful.
A positive drug test simply means use or ingestion of a drug. Even with a quantitation in urine, it is not possible to determine the amount of drug that was used or time of use (other than sometime within the detection window). It may be a first time use or chronic use. Even declining quantitative levels can be misleading due to the effect of urine dilution. With the THC metabolite, it is possible to normalize the ng/mL quantitative values to creatinine (a normal product of muscle metabolism that is always excreted in the urine). The lower the creatinine, the more dilute the urine and the higher the creatinine the more concentrated the urine. By using the THC metabolite to creatinine ratio, it is possible to distinguish new marijuana use from old (gradual elimination). For every THC metabolite positive, we provide you with this creatinine normalized ratio. A ratio of 1.5 or greater between sequential positive test results, separated by one week or more, clearly indicates renewed use of cannabinoids. This is the only application of a quantitative value that has an interpretation. The positive test only indicates the need for clinical evaluation. We are happy to assist you if you need to discuss a specific result and provide guidance. Call today.