Drinking an excessive amount of water before a urine test can significantly affect the accuracy and reliability of the results. This action, often called “dilution,” introduces excess fluid that the kidneys rapidly attempt to excrete. The resulting urine is far less concentrated than a sample produced by a normally hydrated person. This lowered concentration of solutes interferes with how a laboratory measures substances, potentially leading to an inconclusive or invalid test result.
Physiological Markers of Dilution
Laboratories use two primary indicators to determine if a urine sample has been diluted: specific gravity and creatinine levels. Specific gravity measures the density of urine compared to water, reflecting the total concentration of dissolved particles. A normal specific gravity typically falls between 1.003 and 1.030, but a sample is flagged as dilute if the value drops to 1.001 to 1.003, indicating high water content.
Creatinine is a waste product of muscle metabolism that the body excretes at a relatively constant rate. In concentrated urine, creatinine levels typically range from 20 to 400 milligrams per deciliter (mg/dL). Rapidly consuming a large volume of water drastically reduces the concentration of this metabolite.
A creatinine level between 2 mg/dL and 20 mg/dL is the standard range used to classify a specimen as dilute. Since the body produces creatinine consistently, a low measurement provides strong evidence that the sample is overly watered down. This objective measurement challenges the validity of the specimen.
Impact on Diagnostic and Drug Testing
The consequences of dilution differ depending on whether the test is a routine diagnostic urinalysis or a toxicology screen.
Drug Screening
For drug screens, dilution causes drug metabolites to fall below the established cutoff concentration. This can lead to a false negative result, where a person who has used a substance is reported as negative because trace amounts are too low to be detected by the assay.
Diagnostic Urinalysis
Dilution in a diagnostic urinalysis can obscure medically relevant findings, potentially leading to a missed diagnosis. Conditions like infection, kidney disease, or diabetes are identified by the concentration of substances such as protein, glucose, or blood cells. If the sample is too diluted, these indicators may be suppressed to an unobservable level, rendering the result inconclusive and requiring a retest.
For both types of testing, a dilute result causes administrative complications, often delaying medical care or employment decisions. Since the lab cannot confirm the absence or presence of a substance, the collection site must request a retest. This delay is problematic in time-sensitive situations, such as pre-employment screening.
How Laboratories Detect Dilution
Laboratories follow strict protocols to validate every urine specimen before analysis. Validity checks include measuring the specific gravity and creatinine concentration. If both markers fall outside acceptable limits, the sample is immediately flagged and classified.
A sample with a creatinine level between 2 and 20 mg/dL and a specific gravity between 1.0010 and 1.0030 is reported as “dilute.” In workplace drug testing, a dilute result, even if negative for drugs, often triggers an automatic mandatory retest.
If the creatinine level is extremely low (less than 2 mg/dL), the laboratory may classify the sample as “substituted” or “adulterated.” This implies a higher suspicion of deliberate tampering, as such low levels are chemically inconsistent with normal biological processes. Consequences for a substituted or adulterated sample are significantly more severe, often resulting in the sample being treated as equivalent to a positive test result.
Health Risks of Excessive Water Intake
Attempting to dilute a urine sample by rapidly consuming a massive volume of water carries serious physical dangers. The primary risk is hyponatremia, or water intoxication. This occurs when excessive fluid intake overwhelms the kidneys’ ability to excrete water, dangerously diluting sodium in the bloodstream.
Sodium is an electrolyte that regulates water balance; its depletion causes cells to swell. While most cells tolerate minor swelling, brain cells are confined within the skull, and this cellular edema can lead to severe neurological symptoms. Initial signs of hyponatremia include nausea, vomiting, headache, and confusion.
In severe cases, brain swelling can progress rapidly, potentially causing muscle spasms, seizures, coma, and death. Although this is a rare outcome for a healthy individual, consuming large volumes of water to manipulate a test result puts people at risk of this life-threatening electrolyte imbalance.