Weeding Through the Information: Interpreting Laboratory Tests to Determine New vs. Residual Use of Marijuana - Insights (2023)

Expires: March 7, 2025


Print Record of Viewing

Presentation (PDF)


Paul Jannetto, Ph.D.

Associate Professor of Laboratory Medicine and Pathology
Director, Clinical and Forensic Toxicology, Clinical Mass Spectrometry Lab and Metals Lab
Division of Clinical Biochemistry
Mayo Clinic, Rochester, Minnesota



Hello. My name is Dr. Paul Jannetto, and I am the director of the Clinical and Forensic Toxicology Laboratory, the Clinical Mass Spectrometry Laboratory, and the Metals Laboratory here at the Mayo Clinic.


I have nothing to disclose.


The objectives of my talk today are to:

  • Recognize some basic facts on how and why marijuana, or tetrahydrocannabinol, is used and abused.
  • Identify how to determine new vs. residual use of marijuana in a patient.
  • Describe how to calculate the carboxy-tetrahydrocannabinol (carboxy-THC) to creatinine ratio along with a decision ratio and demonstrate its clinical utility.
  • And lastly, to correctly interpret a case study to determine if a patient used marijuana between two different urine collections with Mayo Clinic’s new test.

Marijuana facts1

Marijuana is the most commonly used addictive drug after tobacco and alcohol in the United States.Marijuana refers to the dried leaves, flowers, stems, and seeds from theCannabis sativaorCannabis indicaplant. The plant contains the mind-altering chemical tetrahydrocannabinol, or THC, and other compounds. It is commonly known on the street as: blunt, dope, grass, pot, and weed. One of the main effects that makes it attractive to abusers is the euphoria and intensified sensual and aesthetic feelings.

Marijuana facts continued1,2

Marijuana can be used by many different routes including:

  • It can be mixed into foods or edibles such as brownies, cookies, gummy bears and ingested.
  • People can smoke marijuana in hand-rolled cigarettes called joints, or in pipes, or water pipes called bongs. It can also be smoked in blunts, which are emptied cigars refilled with marijuana.
  • To avoid inhaling smoke, some people are also using vaporizers. You can then inhale the vapor using a liquid marijuana extract.

If the marijuana is smoked or inhaled, the bioavailability is ~10%-25% compared to ingestion which is ~5%-20%. The main difference is that when it is smoked or inhaled, theTHC quickly passes from the lungs into the bloodstream and reaches peak concentration within minutes. The blood then carries the chemical to the brain and other organs throughout the bodyso the user feels the effects quicker compared to the oral route, which it takes ~1-3 hours to reach peak concentrations and feel the effects. This delay has led to many patients overdosing when they consume edibles as they consume more THC since they don’t have the immediate effects.

Another concern is from the use of marijuana extracts. Smoking THC-rich resins extracted from the marijuana plant is on the rise and is called “dabbing.” The extracts come in various forms including hash oil, which is a gooey liquid; wax or budder, which is soft solid like lip balm; and shatter, which is solid. These extracts can deliver extremely large concentrations of THC to a person, causing them to end up in the emergency room. Since these extracts are commonly prepared using butane or lighter fluid, people have also caused fires, explosions, and been badly burned when making these extracts.


While marijuana contains over 60 different cannabinoids, the delta-9 THC is the primary ingredient responsible for its psychoactive effects. In recent decades, the amount of THC in marijuana has been steadily increasing.THC acts on specific brain cell receptors that ordinarily react to natural THC-like chemicals. These natural chemicals play a role in normal brain development and function.

As a result, the use of marijuana can have both short- and long-term effects. Short-term use can lead to impaired motor coordination, altered judgement, impaired short-term memory, hallucinations, and even psychosis at high doses. With chronic heavy usage, marijuana has been shown to alter the brain development, especially when people began using it in their teens. Long-term use can also lead to cognitive impairment, chronic bronchitis, and decreased life satisfaction.

Laboratory testing for marijuana4,5

Urine testing is commonly used to detect marijuana use. Cannabinoid immunoassay methods typically use an antibody directed against one of the metabolites of THC known as carboxy-tetrahydrocannabinol, or carboxy-THC. These commercial immunoassays offer cutoffs of 20 or 50 ng/mL.

Depending on the frequency of marijuana usage, its potency, or THC content, the detection limit of the laboratory assay and other factors, you can still get positive results or detect the carboxy-THC in the urine 30 days or longer once a person stops using marijuana. While the immunoassays for marijuana are targeted against a specific metabolite, they still can have potentially false positive results. For example, the metabolite of an antiviral medication (Sustiva®)used to treat HIV can cross-react and give a “positive” result.

Another question commonly asked is if passive exposure to marijuana will give a positive immunoassay result. While it is possible if using the lower 20 ng/mL cutoff, it really depends on the size and ventilation of the room and the amount of cannabis smoked. In very confined spaces for prolonged exposure, a person could test positive.

Lastly, laboratories also offer definitive tests using gas chromatography or liquid chromatography tandem mass spectrometry that also commonly target the carboxy-tetrahydrocannabinol at lower detection limits.

New vs residual marijuana use6,7

Since marijuana is lipophilic and has a long elimination half-life, it can be detected in urine for weeks to months after stopping usage. As a result, a physician often needs to be able to distinguish between new usage of marijuana vs residual marijuana. It is important since the consequences of multiple positive urine cannabinoid tests could mean discharge from an addiction management program, loss of employment, loss of child custody, or even more severe criminal punishment.

As a result, several studies and predictive models have been published. In these models, investigators recommend using urine creatinine normalized carboxy-tetrahydrocannabinol concentrations at two time points and calculating a decision ratio from these values.

New vs. residual marijuana use continued6,8

In the original model, you normalize the carboxy-THC concentration to the creatinine concentration of the urine to first account for any varying hydration status. You then collect a second urine at least a day or more later and again normalize the carboxy-THC concentration to the creatinine. Next you take the second normalized Carboxy-THC and you divide it by the first normalized carboxy-THC to get a decision ratio. If that ratio was greater than or equal to 0.5 it indicated new usage. However, this only led to 85% accuracy and a high (5.6%) false positive rate. Therefore, it was updated so that if the ratio was actually greater than or equal to 1.5 it indicated new usage of marijuana. This resulted in a much lower false positive rate of 0.1%, but a higher false negative rate.

As a result, Smith and colleagues created an updated model using specific time intervals between the two urine collections from 0-23.9 hours, 24-47.9 hours, 48-71.9 hours, all the way up to seven days later.

New vs. residual marijuana use updated interpretation table6

With this new model, you still take the second creatinine normalized Carboxy-THC (U2) and divide it by the first creatinine normalized carboxy-THC (U1) to get a decision ratio. Next, you determine the time period in hours between the two urine collections. For the most conservative approach to report new marijuana usage, you would then apply the U2/U1 decision ratio to see if it was greater than themaximum listed in the table. However, a more realistic decision ratio with reasonable certainty would be to use the 95% or below limits in the same table. If the U2/U1 ratios are above those limits, it would indicate new usage between those collection time points.

Carboxy-tetrahydrocannabinol (THC-COOH) confirmation and creatinine ratio, random,urine

As a result, Mayo Clinic has created a carboxy-THC and creatinine ratio test in urine to facilitate using these decision ratios to determine new vs. residual use of marijuana. This test will measure both the carboxy-THC using a definitive test method and the urine creatinine. In addition, it will also calculate and report out the creatinine normalized carboxy-THC value.

Clinical case study

So let’s use this new test in an actual clinical case study.

  • In this situation, the physician has a new patient with a known history of marijuana usage.
  • The patient tests positive for THC in the urine with a definitive test but denies continued usage. Instead, they adamantly state their last use was a month ago.
  • Due to its lipophilicity and long elimination half-life, marijuana metabolites can be detected >30 days in both blood and urine tests.
  • Therefore, the physician wants to test the patient for ongoing use of marijuana. They need to be able to distinguish between residual vs. new usage of marijuana. As a result, the physician orders Mayo Clinic’s new test (Mayo test code = THCCR) on two separate dates or urine collections.

For the original collection on Oct. 6, 2021, at 8:00 a.m., we see the patient still tested positive for the carboxy-THC. As a result, a second urine sample was collected on Oct. 7, 2021, at 9:30 a.m. This result was also positive for carboxy-THC. If you just looked at the two carboxy-THC concentrations, you would see that the second collection yielded a higher carboxy-THC concentration, but this should not be interpreted as “new” use of marijuana.

Instead, you need to start by obtaining the two creatinine normalized carboxy-THC values in urine where the collections are 1-7 days apart. As you can see, the Mayo test automatically calculates and reports out the creatinine normalized carboxy-THC ratios.

Case study continued6

Therefore, step #1 is obtaining two creatinine normalized carboxy-THC ratios in urine where the collections are 1-7 days apart.

Step #2 is to then calculate the decision ratio by taking the second creatinine normalized Carboxy-THC (U2) and dividing it by the first creatinine normalized carboxy-THC (U1). In this example, you would take the 300 ng THC-COOH/mg creatinine and divide it by the 481 ng THC-COOH/mg creatinine to get a decision ratio of 0.62.

Step #3 is to then compare this decision ratio to the interpretation table using the time interval between the two urine collections and comparing it to the95% below limits in the same table.

So in this clinical case study, the time in hours between the two urine collections is 25.5 hours and falls between the 24-47.9 hours in the table.

You then compare the decision ratio of 0.62 to the95% below limits in the same row in the table which is 1.01.

Since the decision ratio of 0.62 is less than the 95% below limit of 1.01, this indicates no new usage between the two collection time points.

However, the physician is still suspicious of the patient’s behavior and decides to order a third random urine Carboxy-THC to creatinine ratio (THCCR) test to check if the patient is still abstinent and not using marijuana on Oct. 11.

The physician will now be able to determine if there was new usage between the second collection on Oct. 7, 2021, at 9:30 a.m. and the third collection on Oct. 11, 2021, at 8:30 a.m. Again, we need to start by taking these two creatinine normalized carboxy-THC values and calculating a decision ratio. You cannot just look at the two carboxy-THC concentrations and assume the patient didn’t use since the carboxy-THC concentration was lower (180 ng/mL) in the later collection compared to the 300 ng/mL in the earlier collection.

So in this example, we need to again start by calculating the decision ratio by taking the most recent creatinine normalized Carboxy-THC (the third collection which is going to be our U2) and divide it by the previous in this case (in this case that was the second collection which will be our U1) creatinine normalized carboxy-THC concentration.

Therefore, you would take the 514 ng THC-COOH/mg creatinine and divide it by the 300 ng THC-COOH/ mg creatinine to get a decision ratio of 1.71.

Step #3 is then to compare this decision ratio to the interpretation table using the time interval between the two urine collections and comparing it to the95% below limits in the same table.

As a result, we need to determine the time in hours between the third and second urine collections, which was 95 hours (just shy of 4 days) and falls between the 72-95.9 hours in the table.

You then compare the decision ratio of 1.71 to the95% below limits in the same row in this table, which is 0.595.

Since the decision ratio of 1.71 is greater than the 95% below limit of 0.595, this indicates new usage between the two collection time points.


In summary, marijuana use can have a wide range of health effects, including:

  • Hallucinations and paranoia.
  • Breathing problems.
  • Possible harm to a fetus's brain in pregnant women.
  • Impaired body movement.
  • Difficulty with thinking and problem-solving.
  • Impaired memory and learning.

It is important to distinguish between new vs. residual use of marijuana due to its lipophilicity and long elimination half-life.

Collecting two urine samples and using Mayo Clinic’s THCCR test can assist with identifying new vs. residual use of marijuana.

Thank You

Thank you for your attention. I hope you found this presentation useful. If you have any questions, please contact Mayo Clinic Laboratories.


  1. National Institute on Drug Abuse. Marijuana Retrieved from http://www.drugabuse.gov/publications/drugfacts/marijuana on January 21, 2022.
  2. Borgelt LM, Franson KL, Nussbaum AM, Wang GS. The pharmacologic and clinical effects of medical cannabis. Pharmacotherapy. 2013 Feb;33(2):195-209.
  3. Volkow ND, Baler RD, Compton WM, Weiss SR. Adverse health effects of marijuana use. N Engl J Med. 2014 Jun 5;370(23):2219-27.
  4. Mulé SJ, Lomax P, Gross SJ. Active and realistic passive marijuana exposure tested by three immunoassays and GC/MS in urine. J Anal Toxicol. 1988 May-Jun;12(3):113-6.
  5. elSohly MA, Jones AB. Drug testing in the workplace: could a positive test for one of the mandated drugs be for reasons other than illicit use of the drug? J Anal Toxicol. 1995 Oct;19(6):450-8.
  6. 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.
  7. Schwilke EW, Gullberg RG, Darwin WD, et al. Differentiating new cannabis use from residual urinary cannabinoid excretion in chronic, daily cannabis users. Addiction. 2011 Mar;106(3):499-506.
  8. Huestis MA and Cone EJ. Differentiating new marijuana use from residual drug excretion in occasional marijuana users. J Anal Toxicol 1998;22:445-54.


Contact us:mcleducation@mayo.edu

Photo Credit



Top Articles
Latest Posts
Article information

Author: Moshe Kshlerin

Last Updated: 12/26/2023

Views: 5779

Rating: 4.7 / 5 (77 voted)

Reviews: 84% of readers found this page helpful

Author information

Name: Moshe Kshlerin

Birthday: 1994-01-25

Address: Suite 609 315 Lupita Unions, Ronnieburgh, MI 62697

Phone: +2424755286529

Job: District Education Designer

Hobby: Yoga, Gunsmithing, Singing, 3D printing, Nordic skating, Soapmaking, Juggling

Introduction: My name is Moshe Kshlerin, I am a gleaming, attractive, outstanding, pleasant, delightful, outstanding, famous person who loves writing and wants to share my knowledge and understanding with you.