Both the creatinine and SG must be below satisfactory numbers in order to be ruled diluted. Samples are ruled "substituted" when the creatinine and SG levels are so low and/or divergent that its not consistent with human urine.

Refractometry is used as the confirmatory methodology for SG, while Colorimetry is used for creatinine. Details of both methodologies and more, can be found here:
http://www.workplace.samhsa.gov/Drug...ary%202005.pdf