A urine drug screen detects metabolites, which are compounds produced when the body breaks down a substance. For cannabis use, the target is the non-psychoactive metabolite 11-nor-9-carboxy-delta-9-tetrahydrocannabinol (THC-COOH). Laboratories establish a specific concentration threshold, known as a cutoff level, to determine a positive result. The practice of excessive fluid intake before a test is a strategy intended to reduce the concentration of these metabolites, potentially pushing the levels below the established cutoff.
The Mechanism of Dilution and THC Metabolites
The human body’s renal system works to maintain the balance of water and solutes in the blood. When a person consumes a large volume of fluid, the body registers an excess of water, triggering a physiological response. Specifically, the pituitary gland reduces the secretion of antidiuretic hormone (ADH), which normally signals the kidneys to conserve water.
With less ADH present, the kidneys become less permeable to water, preventing its reabsorption back into the bloodstream. This causes a rapid increase in the volume of water excreted as urine. The influx of water increases the clearance rate of water-soluble compounds, including the THC-COOH metabolites being processed for excretion.
This process effectively lowers the concentration of the metabolites by dissolving them into a much larger volume of liquid. For a urine test to be reported as positive, the concentration of the THC-COOH metabolite must meet or exceed the laboratory’s confirmation cutoff, often set at 15 nanograms per milliliter (ng/mL). Dilution can lower the concentration below this cutoff, resulting in a negative test even if the substance was recently present. However, this intentional manipulation triggers secondary laboratory checks designed to flag the sample’s validity.
Laboratory Markers for Identifying Diluted Samples
Testing facilities understand that drug and metabolite concentrations can be affected by hydration status, so they employ specific measures to check the integrity of the sample. Two primary markers are used to assess the concentration and validity of a urine specimen: creatinine and specific gravity. These markers act as internal benchmarks because they are consistently produced and excreted at predictable rates by the body.
Creatinine
Creatinine is a waste product generated from the breakdown of creatine phosphate in muscle tissue, and its excretion rate is relatively constant. A normal, concentrated urine sample typically contains creatinine levels well above 20 milligrams per deciliter (mg/dL). If the sample is dilute, the creatinine concentration will fall significantly. Laboratories commonly classify a urine specimen as dilute if the creatinine level is greater than or equal to 2 mg/dL but less than 20 mg/dL.
Specific Gravity (SG)
The second marker, specific gravity (SG), measures the density of the urine sample compared to pure water, reflecting the total concentration of all dissolved solids, or solutes. The SG of a normal urine sample typically ranges between 1.005 and 1.030. A highly dilute sample will have an SG much closer to 1.000. Under federal guidelines, a sample is flagged as dilute if the specific gravity is greater than 1.0010 but less than 1.0030, in addition to the low creatinine level.
If the concentration of these two markers falls below the most extreme thresholds, the specimen may be classified not just as dilute, but as substituted or invalid. For instance, a creatinine level below 2 mg/dL, coupled with an SG less than 1.0010, suggests that the sample is physiologically impossible for human urine and may have been tampered with or replaced entirely. By testing for both creatinine and specific gravity, the laboratory determines if the low metabolite count is a result of dilution rather than true abstinence.
Interpreting Diluted Test Results
When a sample is flagged as dilute, the result is reported to a Medical Review Officer (MRO), who is responsible for interpreting the findings and consulting with the donor. The MRO differentiates between two main outcomes based on the concentration of the drug metabolite, even within a dilute sample.
The first result is a “Dilute Negative,” meaning the sample was confirmed dilute based on the low creatinine and specific gravity, but the THC-COOH concentration was below the required cutoff level. While a dilute negative technically means no drug metabolite was detected above the threshold, many regulatory bodies and employers do not treat it as a standard negative.
The dilution raises suspicion that the donor attempted to mask drug use. Consequently, the employer or agency may require an immediate retest, which is sometimes conducted under direct observation to prevent further attempts at dilution or substitution.
The second outcome is an “Invalid” or “Substituted” result, which carries much more severe administrative consequences. If the creatinine and specific gravity levels suggest a total substitution of the sample, or if adulterants are detected, the MRO will often report the result as a refusal to test.
In most workplace or legal testing scenarios, a refusal to test is treated with the same severity as a confirmed positive drug test result. Therefore, while dilution can successfully reduce the concentration of THC-COOH below the cutoff, the resulting flag on the sample’s integrity often negates the intended benefit and introduces a significant administrative risk for the donor.