The practice of urine drug testing is common in employment and legal contexts. A frequent question is whether drinking excessive amounts of water prior to the test can dilute the urine. High fluid intake lowers the concentration of drug metabolites below detectable levels, aiming for a negative result. However, testing facilities use specific scientific measures to identify and flag diluted samples.
The Science Behind Sample Dilution
The kidneys filter waste products and regulate fluid balance, determining urine composition. Consuming a large volume of water causes the kidneys to rapidly increase water excretion to maintain internal balance. This surge dilutes the urine, reducing the concentration of all components, including drug metabolites.
Laboratories assess urine concentration using two primary measurements. Specific Gravity measures the density of the sample compared to water, reflecting the total dissolved solids present. Creatinine is a metabolic waste product produced constantly by muscle breakdown and excreted by the kidneys. Both substances must be present within established normal ranges for human urine.
Excessive water consumption significantly lowers the concentration of both markers. When the kidneys flush out large amounts of fluid, Specific Gravity decreases because the ratio of water to dissolved solids increases. The rapid fluid turnover also reduces the concentration of excreted creatinine far below expected levels. This physiological response forms the basis for how testing facilities detect dilution attempts.
How Testing Facilities Identify Dilution
Testing facilities use established scientific thresholds, often guided by federal standards, to formally classify a sample as dilute. A specimen is deemed “dilute” if the creatinine concentration is below a specific value, typically less than \(20 \text{ mg/dL}\). This low level suggests the sample does not represent normal human physiological output.
The Specific Gravity must also fall below a set minimum value, usually less than \(1.003\). When both creatinine and Specific Gravity measurements fall below these concurrent thresholds, the laboratory confirms the sample as dilute. This combined validity check ensures the integrity of the testing process.
A diluted sample, caused by excessive water intake, is low in natural markers but chemically normal. An adulterated sample has a foreign substance added, like bleach or soap, to interfere with the test chemistry. Adulteration is identified through abnormal pH levels or the presence of non-natural compounds. Testing facilities have distinct protocols for handling both situations, as both invalidate the original test result.
Consequences of a Diluted Result
When a laboratory determines a specimen is dilute, the result is communicated as “Negative-Dilute,” “Invalid,” or “Inconclusive.” A “Negative-Dilute” result means no drug metabolites were detected, but the specimen was too watered down for a valid negative finding. Conversely, a “Positive-Dilute” result means metabolites were detected despite the dilution, and this is treated as a standard positive test.
For a “Negative-Dilute” finding, the standard procedure is often to require a re-test. This re-test is usually conducted quickly, sometimes under stricter conditions, to prevent further dilution attempts. The administrative action taken is determined by the requesting organization, such as an employer or court system, not the laboratory.
Some organizational policies may treat a diluted specimen as equivalent to a refusal to test, especially if there is a pattern of diluted results. The consequence of a diluted result is determined by the specific policy of the testing program, not the science. Attempting to dilute a sample does not guarantee a negative result and almost always triggers scrutiny and administrative action.