Urine alcohol screening is a common method used to monitor abstinence or detect recent alcohol use. These sophisticated tests do not typically look for active alcohol (ethanol), which clears from the body quickly. Instead, they focus on specific breakdown products known as metabolites, primarily ethyl glucuronide (EtG) and ethyl sulfate (EtS). The high sensitivity of these metabolite tests has led to the misconception that drinking large volumes of water can “flush” the system and defeat the screening process. Modern laboratory practices, however, are specifically designed to identify this dilution tactic, making it an ineffective strategy for masking alcohol consumption.
Alcohol Metabolism and Excretion
The human body processes nearly all ingested ethanol, with 90 to 95 percent being metabolized in the liver. This primary pathway converts ethanol into acetaldehyde, which is further broken down into acetic acid and eventually eliminated. A much smaller fraction is processed through non-oxidative metabolism.
This minor pathway is responsible for creating the water-soluble compounds EtG and EtS. These metabolites are then excreted through the kidneys and into the urine. Because the liver’s metabolic rate is relatively fixed, the speed at which these metabolites are generated and eliminated cannot be significantly altered by fluid intake. EtG and EtS provide a much longer detection window—up to several days—compared to the mere hours ethanol is detectable.
The Effect of Excessive Water Intake on Urine Concentration
Excessive water intake aims to dilute the urine sample, thereby lowering the concentration of EtG and EtS below the test’s cutoff threshold. Drinking a large volume of water in a short period temporarily increases the amount of solvent (water) in the urine. This transiently lowers the ratio of the metabolite to the total fluid volume.
While the concentration can be lowered, the total amount of EtG and EtS that the body must excrete remains unchanged. The kidneys quickly work to maintain fluid balance, meaning the dilution effect is temporary, often lasting only two to six hours. Laboratories correct for dilution by calculating the ratio of the metabolite concentration to another stable urinary component, revealing the true level of alcohol exposure.
Laboratory Measures to Detect Sample Tampering
Laboratories employ specific scientific measures, known as validity checks, to determine if a urine sample has been diluted. The primary focus is on two markers that reflect the natural density and concentration of urine: specific gravity and creatinine. These markers provide evidence of whether the sample is water-logged, regardless of the alcohol metabolite concentration.
Creatinine is a waste product generated consistently by muscle metabolism and filtered out by the kidneys. It is produced at a constant rate, making it an excellent internal reference marker. A sample is flagged as dilute if creatinine concentration falls below 20 milligrams per deciliter (mg/dL), indicating an abnormally high proportion of water.
Specific gravity measures the total concentration of dissolved particles in the urine compared to pure water. Normal human urine typically ranges between 1.003 and 1.030. A sample is defined as diluted if its specific gravity is below 1.0030, which correlates with low creatinine levels.
Potential Health and Testing Consequences
Attempting to dilute a sample by consuming large amounts of water carries testing repercussions and serious medical risks. A sample flagged as “dilute” due to low creatinine and specific gravity is often not accepted as a valid negative result. It is treated as an invalid test or a “dilute negative,” typically resulting in a mandatory, unannounced re-test.
The re-test is performed under stricter supervision to prevent further tampering attempts. Consuming several liters of water quickly can lead to a severe, though rare, medical emergency called water intoxication. This condition causes hyponatremia, where rapid water intake overwhelms the kidneys and dilutes the body’s sodium levels. The resulting imbalance can cause brain swelling, leading to confusion, severe headache, seizures, and in rare cases, death.