Urine drug screening is a frequent method used for pre-employment, legal, or athletic testing. While the intention behind adding water is to dilute drug metabolites, this form of tampering, known as external dilution or adulteration, is easily detectable. Modern testing protocols are designed to ensure sample integrity and have multiple layers of defense to flag compromised samples.
Why Directly Adding Water Fails
The most immediate indicator that water has been added externally is the sample’s temperature. A freshly collected urine specimen is expected to be within a narrow range, typically between 90°F and 100°F (32°C to 38°C). Collection cups are equipped with a temperature strip that a collector reads immediately after the sample is provided.
Adding room-temperature or cold water instantly drops the temperature outside this acceptable window. The collector must check the temperature within four minutes, and a reading outside the 90°F to 100°F range is an immediate flag for tampering or substitution. This discrepancy alone is enough to invalidate the test and trigger a retesting protocol.
The collection process is also designed to prevent donors from accessing external water sources. Facilities employ strict chain-of-custody protocols, which may involve removing outer garments and placing a bluing agent in the toilet water to prevent substitution. Even non-observed collections occur in a secure environment where access to running water is restricted.
A visually compromised sample raises suspicion before it reaches the laboratory. Excessive dilution makes the urine appear abnormally clear or colorless, which a trained collector notes on the chain-of-custody form. This visual observation, combined with an out-of-range temperature, usually results in the sample being rejected on the spot as an adulterated specimen.
Laboratory Metrics for Detecting Dilution
When a sample reaches the laboratory, it undergoes Specimen Validity Testing (SVT), which measures the concentration of natural components in the urine. These tests determine if the specimen is genuine. Two primary metrics are used to detect dilution: specific gravity and creatinine levels.
Specific gravity (SG) measures the density of the urine compared to water, which has an SG of 1.000. Normal human urine falls between 1.003 and 1.030. A specimen is reported as “dilute” if its specific gravity is less than 1.0030, indicating an abnormally high proportion of water relative to natural waste products.
Creatinine is a waste product of muscle metabolism excreted at a relatively constant rate. Certified laboratories, following guidelines set by the Substance Abuse and Mental Health Services Administration (SAMHSA), use creatinine levels to assess concentration. A sample is considered dilute if the creatinine concentration is less than 20 milligrams per deciliter (mg/dL). If the level drops below 2.0 mg/dL, it is considered physiologically impossible for human urine and may be flagged as substituted or adulterated.
Laboratories also test for chemical adulterants, such as nitrites or glutaraldehyde, which interfere with drug screening. SG and creatinine remain the standard measures for detecting water-based dilution, requiring a specimen to fail both criteria to be formally reported as dilute.
The Outcome of Submitting a Diluted or Invalid Sample
The consequences of submitting a compromised sample depend on whether the test is classified as “dilute,” “invalid,” or “adulterated.” An “adulterated” or “substituted” result, often following an attempt to add water directly, is treated as an immediate refusal to test. Under Department of Transportation (DOT) and many workplace policies, a refusal to test is equivalent to a positive result, leading to severe implications like immediate termination or a mandatory return-to-duty process.
A sample that is merely “dilute,” often due to excessive fluid intake, is not an automatic failure. If the lab finds no drug metabolites, it is reported as a “Negative Dilute.” The employer or mandated program then requires a retest under supervised conditions to ensure the second sample is concentrated enough for accurate analysis.
If drug metabolites are found in a dilute sample, it is reported as a “Positive Dilute,” which is treated as a positive test result. When the laboratory cannot confirm the presence or absence of a substance due to interference, the result is reported as “Invalid.” An invalid result often triggers a requirement for a re-collection under direct observation, as the sample’s integrity could not be guaranteed.