Optimized evaluations to accurately monitor marijuana use

Urine drug testing to distinguish new from residual use

Cannabis, commonly known as marijuana, refers to products derived from Cannabis sativa or Cannabis indica plants, which contain the psychoactive compound delta-9-tetrahydrocannabinal (THC). Exposure to THC can cause euphoria, intensified sensations, altered pain sensitivity, and a range of other effects, but when misused can lead to intoxication, withdrawal, and biopsychosocial issues.1

Despite varying state legalization initiatives, marijuana is categorized by the Drug Enforcement Agency as a Schedule 1 compound. After alcohol and tobacco, marijuana is the most commonly used addictive drug in the United States. Among individuals affected by an illicit drug use disorder, marijuana is the most frequently misused substance. As a result, many professional practice guidelines recommend the use of urine drug monitoring (UDM) to identify illicit use and to support patient recovery.

Since marijuana is lipophilic and has a long elimination half-life, THC and its major metabolite, delta-9-carboxy-tetrahydrocannabinal (THC-COOH), can be detected in urine for weeks to months after last usage. To accurately distinguish new from residual use, THC-COOH and creatinine ratios of specimens collected over two known time points should be compared to determine if new marijuana use has occurred.

By the numbers

48.2 Million

Americans used marijuana at least once in 2019.2


average annual increase in cannabis-associated ER visits between 2006-2015.3

4 Million

people affected by marijuana use disorder in 2016.2

Tailored testing to clarify new marijuana use

Traditional laboratory testing often fails to distinguish between new and residual drug use, leading to misinterpretation of positive test results. Because multiple positive urine test results can lead to discharge from a drug treatment program, loss of employment, loss of child custody, and other negative consequences, differentiating between new and past use is critical.

Propelled by research highlighting a better approach to identifying marijuana use, Mayo Clinic Laboratories has created a carboxy-THC and creatinine ratio urine test that facilitates use of decision ratios to determine new versus residual use of marijuana. This approach uses urine creatinine- normalized carboxy-THC concentrations gathered at two time points and calculates a decision ratio from those values. Through analysis and calculation of the decision ratio, test reports clarify patients’ new versus residual drug use.

Key testing

THCCR  | Carboxy-Tetrahydrocannabinol (THC-COOH) Confirmation and Creatinine Ratio, Random, Urine


  • Analyzes and reports the creatinine-normalized THC-COOH value.
  • Facilitates calculation and use of decision ratios to determine new versus residual drug use using urine creatinine-normalized carboxy-tetrahydrocannabinol concentrations gathered at two time points.

Identify new cannabis use through decision ratios

Our research-driven assay begins with obtaining two creatinine-normalized carboxy-THC ratios in urine where the collections are 1–7 days apart.

Next, using the creatinine-normalized THC-COOH ratios from the results above, calculate the decision ratio by taking the second creatinine-normalized THC-COOH result (U2) and dividing it by the first creatinine-normalized carboxy-THC (U1).

Finally, compare the decision ratio to an interpretation table using the time interval between the two collections. For the most conservative approach to report new marijuana usage, compare the U2/U1 decision ratio to see if it was greater than the maximum listed in the table. However, a more realistic decision ratio with reasonable certainty would be to use the 95% below limits in the same table. If the U2/U1 ratios are above these limits, it would indicate new usage between those collection time points.4

Enhanced detection of new versus residual marijuana use

Paul Jannetto, Ph.D., discusses Mayo Clinic Laboratories’ new targeted marijuana monitoring assay, highlighting the research behind and advantages of using carboxy-THC and creatinine ratio urine testing.

Additional testing

When the tetrahydrocannabinol-to-creatinine ratio is not required to distinguish new versus residual use, consider THC testing as part of a controlled substance monitoring profile or an individual confirmatory test.

Ensuring accuracy of result interpretation

Analysis of urine drug testing results by our experts, who apply the most up-to-date scientific data, enables further transparency and clear answers that illuminate patients’ drug use. Test results are accompanied by interpretive reports the clearly identify the presence of prescribed, nonprescription, and illicit drugs. The inclusion of testing limitations, details on the window of drug detection, and information on the presence of pharmaceutical impurities and metabolic breakdowns of parent drugs in the reports provides important insights to guide treatment decisions.

Learn more about interferences and impurities that might impact controlled substance simple screening results.

Pharmacokinetics of Illicit Substances

Substances that can interfere with illicit substances screening

Learn more about how to order these tests at your institution.



  1. Patel J, Marwaha R. Cannabis Use Disorder. The National Center for Biotechnology Information. Updated February 7, 2022. Accessed March 29, 2022. www.ncbi.nlm.nih.gov/books/NBK538131
  2. Substance Abuse and Mental Health Services Administration. Key Substance Use and Mental Health Indicators in the United States: Results from the 2016 National Survey on Drug Use and Health. Accessed March 29, 2022. www.samhsa.gov/data/sites/default/files/NSDUH-FFR1-2016/NSDUH-FFR1-2016.htm
  3. Roehler D, et al. Trends and characteristics of cannabis-associated emergency department visits in the United States, 2006–2018. Drug Alcohol Depend. 2022 Mar 1;2022;232.
  4. Smith ML, Barnes AJ, Huestis MA. Identifying new cannabis use with urine creatinine-normalized THCCOOH concentrations and time intervals between specimen collections. J Anal Toxicol. 2009 May;33(4):185-9.