Creatinine is checked with all GC/MS.
Actually Web, the GC/MS is not used to check creatinine levels. The GC/MS is used only to confirm drug positives and two types of adulterants: Pyridine and Glutaraldehyde. Creatinine levels are checked using colorimetry (the typical validity dipstick/cup/cassette) while Refractometry is used for checking the specific gravity. Colorimetry can also be used for SG as the initial analysis.

Source: http://www.workplace.samhsa.gov/Drug...ary%202005.pdf