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Interpretive Handbook

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Test 8898:
Tetrahydrocannabinol (THC) Confirmation, Urine

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Delta-9-tetrahydrocannabinol is the active agent of the popularly abused street drug, marijuana.

 

Following consumption of the drug, either by inhalation or ingestion, it is metabolized to a variety of inactive chemicals, 1 of them being delta-9-tetrahydrocannabinol carboxylic acid.

 

The immunochemical procedure used to screen for tetrahydrocannabinol (THC) as part of #8248 "Drug Abuse Survey, Urine" is designed to cross-react with THC carboxylic acid.

 

In almost all medico-legal cases and in screening of employees, or when the patient adamantly denies THC use and the immunochemical test is positive, confirmation of the result by gas chromatography-mass spectrometry (GC-MS) and EIA are required.

Useful For Suggests clinical disorders or settings where the test may be helpful

Detection and confirmation of drug abuse involving delta-9-tetrahydrocannabinol (marijuana)

Interpretation Provides information to assist in interpretation of the test results

The presence of tetrahydrocannabinol carboxylic acid (THC-COOH), a major metabolite of delta-9-tetrahydrocannabinol, in urine at concentrations >15 ng/mL is a strong indicator that the patient has used marijuana.

 

The metabolite of marijuana (THC-COOH) has a long half-life and can be detected in urine for more than 7 days after a single use. The presence of THC-COOH in urine >100 ng/mL indicates relatively recent use, probably within the past 7 days. Levels >500 ng/mL suggest chronic and recent use. Chronic use causes accumulation of THC and THC-COOH in adipose tissue such that it is excreted into the urine for as long as 30 to 60 days from the time chronic use is halted.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

No significant cautionary statements

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Negative

Positives are reported with a quantitative GC-MS result.

 

Cutoff concentrations:

IMMUNOASSAY SCREEN

<20 ng/mL

 

THC CARBOXYLIC ACID BY GC-MS

<3 ng/mL

Clinical References Provides recommendations for further in-depth reading of a clinical nature

Moyer TP, Palmen MA, Johnson P, et al:  Marijuana testing-how good is it? Mayo Clin Proc 1987;62:413-417