Distinguish new from residual THC use

Identify new use and confirm cessation through decision ratios

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. 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 created a carboxy-THC and creatinine ratio urine test that facilitates use of decision ratios to determine new versus residual marijuana use. 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.
  • Identifies new cannabis use through decision ratios.

Compare decision ratios to confirm THC cessation

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.9

Accurately calculate and interpret THCCR results

Our clinically supported testing is developed by a team of industry-leading toxicology experts who are integrated with the clinical practice at Mayo Clinic and available to provide guidance and explain unexpected results.

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.

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


  1. 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