What is a diluted sample in urine drug testing?

By Adrianna MacPherson | Last updated: April 6, 2023

In simplest terms, in urine drug testing, a urine sample is dilute if it contains higher than expected levels of water.

Normal human urine contains creatinine at a concentration of 20 mg/dL or higher. If a sample has a creatinine level of less than 20 mg/dL then it may be deemed dilute. The level of creatinine is often determined during specimen validity testing. This testing is specifically designed to detect dilution, substitution or adulteration of urine samples.

Additional testing, including evaluation of the sample’s specific gravity, may also be conducted to confirm a dilute finding. The final determination of a specimen’s status will depend on the results of this further testing and the level of dilution.

A finding that a sample is dilute is not always an indication of tampering. It is possible for a test subject to drink large enough amounts of fluid sufficient to inadvertently, or intentionally, dilute a urine specimen. Medications such as diuretics can also cause a sample to be dilute.

Of course, physical dilution of the sample through the intentional addition of fluids to the sample is also a possibility. Most test collection sites prevent test subjects from having access to liquids during a specified period before sample production to reduce the risk that intentional tampering will take place.

A finding that a sample has been diluted may result in a negative or invalid test result. If a test sample yields a negative result, but is dilute, Department of Transportation (DOT) guidelines permit the employer to obtain a second sample. However, the employer is not obligated to do so and may accept a negative dilute test result as a valid negative result.

It is also possible for a urine test to indicate a positive result despite dilution. A dilute positive result is to be treated as a verified positive result under DOT guidelines and retesting is not permitted.

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