A negative-dilute result is a negative DOT drug test in which the laboratory also found the urine specimen to be dilute. 49 CFR 40.3 defines a dilute specimen as "a urine specimen with creatinine and specific gravity values that are lower than expected for human urine."
Under 49 CFR 40.155, the Medical Review Officer (MRO) reports to the Designated Employer Representative (DER) that the specimen is dilute and explains the employer's obligations and choices under 49 CFR 40.197. That section sets two paths. When the creatinine concentration is at least 2 mg/dL but no more than 5 mg/dL, the MRO directs an immediate recollection under direct observation, and the employer conducts it immediately (49 CFR 40.155(c), 40.197(b)(1)). When the creatinine concentration is greater than 5 mg/dL, the employer may, but is not required to, direct the employee to take another test immediately, and that recollection is not directly observed unless another basis for direct observation applies.
Section 40.197 states that an employer that uses optional retests must treat all employees the same and inform employees of its policy in advance, although it may set different policies for different types of tests. The result of the directed test, not the earlier test, is the test result of record. Declining a directed test is a refusal to test. A positive result that is also dilute is treated as a verified positive.
Related topics are covered in When Is a DOT Urine Collection Directly Observed? and What Counts as Refusing a DOT Drug Test?.
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