49 CFR §40.88

1. (a)As a laboratory, you must consider the primary specimen to be dilute when:
   1. (1)The creatinine concentration is greater than or equal to 2 mg/dL but less than 20 mg/dL, and
   2. (2)The specific gravity is greater than 1.0010 but less than 1.0030 on a single aliquot.
2. (b)As a laboratory, you must consider the primary specimen to be substituted when the creatinine concentration is less than 2 mg/dL and the specific gravity is less than or equal to 1.0010 or greater than or equal to 1.0200 on both the initial and confirmatory creatinine tests and on both the initial and confirmatory specific gravity tests on two separate aliquots.