Carboxy-THC (THC-COOH) is the primary, inactive breakdown product of THC, central to understanding cannabis drug screen results. Standard drug tests, particularly those involving urine and blood, focus on detecting this specific metabolite rather than the psychoactive compound itself. This approach determines if a person has been exposed to cannabis, making the concentration of THC-COOH the main indicator of past use. The interpretation of a “high level” relies heavily on standardized thresholds used by testing laboratories and the underlying biological processes of metabolism and storage.
The Role of Carboxy-THC in Drug Testing
When tetrahydrocannabinol (THC) enters the body, it is processed primarily in the liver. Enzymes convert THC into intermediate compounds, starting with 11-hydroxy-THC (11-OH-THC), which is also psychoactive. The body then oxidizes 11-OH-THC into the final, inactive form, Carboxy-THC. This compound is highly lipid-soluble, meaning it readily dissolves in fat. THC-COOH is stored in fat tissues and released slowly, making it detectable long after the psychological effects of cannabis have faded. Testers focus on Carboxy-THC because its prolonged presence provides a much longer window of detection than active THC, which clears quickly.
Understanding Concentration Units and Testing Thresholds
The level of Carboxy-THC is measured in nanograms per milliliter (ng/mL). A “high level” is defined relative to established cut-off thresholds, such as those set by the Substance Abuse and Mental Health Services Administration (SAMHSA) for urine testing. The testing process involves two stages.
The initial screening test uses an immunoassay with a cut-off level of 50 ng/mL. If a sample meets or exceeds this level, it is considered a presumptive positive and moves to the second, more precise stage. The confirmatory test uses highly specific methods like Gas Chromatography/Mass Spectrometry (GC/MS). This test uses a lower cut-off level of 15 ng/mL. A confirmed positive result requires the Carboxy-THC concentration to meet or exceed this 15 ng/mL threshold.
A “high level” is anything significantly above the 15 ng/mL confirmation threshold. Concentrations ranging from 100 ng/mL to 300 ng/mL or higher are considered high. These levels reflect very recent use, heavy consumption, or chronic, frequent use. For instance, studies on heavy users have shown plasma concentrations reaching up to 284 ng/mL. These elevated concentrations indicate the body is actively processing and excreting a large amount of stored metabolites, suggesting greater exposure than a level barely above 15 ng/mL.
Biological Factors Affecting Carboxy-THC Levels
The measured concentration of Carboxy-THC is not solely determined by the amount of cannabis consumed, as results can vary widely between individuals. Frequency and potency of use are major determinants; a person who uses potent products daily will accumulate higher levels of the metabolite than an occasional user. This chronic exposure leads to greater storage in body tissues.
Individual metabolic rate plays a significant role in how quickly THC-COOH is produced and eliminated. People with faster metabolisms clear the metabolite more rapidly, resulting in lower concentrations compared to those with slower metabolisms. Body composition is another influential factor because Carboxy-THC is lipophilic. Individuals with a higher percentage of body fat store more of the fat-soluble metabolite, leading to higher concentrations and a slower release rate.
Hydration status also directly affects the concentration reading through a dilution effect. A highly diluted urine sample can temporarily lower the ng/mL measurement, potentially causing a falsely low reading. Laboratories often measure creatinine concentration to check for excessive dilution.
Clearance Time and Long-Term Detection
The long detection window of Carboxy-THC is directly related to its storage in fat tissue. After the initial rapid excretion phase, the metabolite is slowly released back into the bloodstream from fat stores, where it is then excreted. This slow release allows for long-term detection.
For infrequent users, Carboxy-THC is typically detectable in urine for three to four days using the standard 50 ng/mL cutoff. However, clearance time is drastically extended in chronic, heavy users. For these individuals, the constant re-release of the stored metabolite can keep concentrations above the 15 ng/mL confirmation threshold for weeks. In documented cases involving heavy daily use, the metabolite has remained detectable for a month or longer, with rare instances extending up to 77 days.