A diluted urine sample can technically result in a false negative on a drug test. A diluted sample contains a higher-than-normal proportion of water, which weakens the concentration of all solutes, including drug metabolites. This reduction can push the drug byproduct below the detection threshold, leading to a negative result that does not accurately reflect the substance’s presence. However, modern laboratories use strict validity checks to identify and flag these diluted samples, preventing them from being reported as a standard negative result.
Understanding Urine Concentration and Dilution
The body maintains a stable internal environment by regulating water balance, a process largely controlled by the kidneys. When a person drinks a large volume of fluids, the body must excrete the excess water. The kidneys respond to this excess hydration by minimizing the release of antidiuretic hormone (ADH), which normally signals the kidney tubules to retain water.
With low ADH, the kidney tubules become less permeable, allowing the excess fluid to pass through the system. This results in a large volume of pale urine with a low concentration of dissolved particles. The high water volume acts as a powerful solvent, diluting the concentration of waste products and metabolites present in the urine. This physiological response is the core mechanism leading to a diluted sample.
How Test Cutoffs Lead to False Negatives
Drug testing relies on the “cutoff threshold,” which is the minimum concentration of a drug metabolite required for a positive report. These thresholds are established to minimize false positive results from environmental exposure or ingestion of small amounts of a substance. The testing equipment registers a positive result only if the metabolite concentration meets or exceeds this predetermined cutoff level.
If a person uses a substance and then drinks a large amount of water, the resulting dilution lowers the concentration of drug metabolites. Even though metabolites are present, their concentration may fall just below the established cutoff threshold. When this occurs, the laboratory instrument registers the result as “negative,” creating a false negative reading that masks the substance’s original presence.
Identifying Manipulated Samples in the Lab
Laboratories use specific scientific markers to determine if a urine sample is physiologically normal or diluted. The two primary markers used for this integrity check are creatinine concentration and specific gravity. Creatinine is a stable waste product of muscle metabolism consistently filtered by the kidneys, making its concentration a reliable indicator of the sample’s overall concentration.
A normal urine sample has a creatinine concentration above 20 milligrams per deciliter (mg/dL). A sample is flagged as dilute if its creatinine level is between 2 and 20 mg/dL. Specific gravity (SG) measures the density of the urine compared to water, reflecting the total concentration of dissolved solids. Normal urine has an SG between 1.002 and 1.030, while a diluted sample has an SG of 1.003 or lower.
What Happens When a Sample Is Flagged as Diluted
When a sample is identified as diluted due to low creatinine and specific gravity, it is not reported as a standard negative result. The result is reported as “Negative-Dilute” or, in certain contexts, “Invalid.” A Negative-Dilute result means no drug metabolites were detected above the cutoff level, but the sample’s integrity was compromised by excessive water content.
The consequence of a diluted result is usually a requirement for the donor to undergo an immediate retest. This second test is often conducted under stricter supervision to ensure sample integrity. For regulatory testing programs, a donor who repeatedly provides a diluted sample may face consequences similar to a refusal to test.